Kaposi's sarcoma is significantly more common in people with AIDS than in the general population or other immunosuppressed groups. Among those with AIDS reported to the Centers for Disease Control by March 31, 1989, 15% had Kaposi's sarcoma. The risk varied by HIV transmission group, with homosexual or bisexual men having the highest risk (21%), while men with haemophilia had the lowest (1%). Women with partners who were bisexual men had higher risks than those with intravenous drug users. The risk was not consistently related to age or race but varied across the United States, being highest in areas where the AIDS epidemic began. These findings suggest that Kaposi's sarcoma in AIDS patients may be caused by an unidentified sexually transmitted infection. The study analyzed data on Kaposi's sarcoma among AIDS patients reported to the CDC. The expected number of cases was estimated based on incidence rates and the proportion of transplant recipients with Kaposi's sarcoma. The overall risk of Kaposi's sarcoma in AIDS patients was at least 20,000 times greater than in the general population and 300 times greater than in other immunosuppressed populations. Among AIDS patients, the percentage with Kaposi's sarcoma ranged from 1% in haemophilic men to 21% in homosexual or bisexual men. The differences in risk between most HIV transmission groups were statistically significant. Homosexual or bisexual men, regardless of whether they were also intravenous drug users, had substantially higher risks of Kaposi's sarcoma than any other group. People born in Caribbean and African countries whose HIV was acquired by heterosexual contact had the next highest risk, with 6% having Kaposi's sarcoma. Men with haemophilia and women who acquired HIV by heterosexual contact had the lowest risks. The percentage of Kaposi's sarcoma varied by race within each transmission group, but the differences between races were not consistent. Among homosexual or bisexual men, the percentage with Kaposi's sarcoma was twice as high in whites as in blacks. In contrast, among transfusion recipients, the percentage of whites with Kaposi's sarcoma was half that for blacks. Among intravenous drug users, there was a slight and nonsignificant excess among whites compared with blacks. The percentage of Kaposi's sarcoma increased with age up to 35–44 years in homosexual or bisexual men, then declined. The shape of the age curve in these men has changed over time, with the peak moving to progressively older ages. Among adults in other risk groups, there was little variation in the percentage with Kaposi's sarcoma by age. Kaposi's sarcoma is rare before the age of 15 years. The proportion of patients with Kaposi's sarcoma was 1.6% for those aged lessKaposi's sarcoma is significantly more common in people with AIDS than in the general population or other immunosuppressed groups. Among those with AIDS reported to the Centers for Disease Control by March 31, 1989, 15% had Kaposi's sarcoma. The risk varied by HIV transmission group, with homosexual or bisexual men having the highest risk (21%), while men with haemophilia had the lowest (1%). Women with partners who were bisexual men had higher risks than those with intravenous drug users. The risk was not consistently related to age or race but varied across the United States, being highest in areas where the AIDS epidemic began. These findings suggest that Kaposi's sarcoma in AIDS patients may be caused by an unidentified sexually transmitted infection. The study analyzed data on Kaposi's sarcoma among AIDS patients reported to the CDC. The expected number of cases was estimated based on incidence rates and the proportion of transplant recipients with Kaposi's sarcoma. The overall risk of Kaposi's sarcoma in AIDS patients was at least 20,000 times greater than in the general population and 300 times greater than in other immunosuppressed populations. Among AIDS patients, the percentage with Kaposi's sarcoma ranged from 1% in haemophilic men to 21% in homosexual or bisexual men. The differences in risk between most HIV transmission groups were statistically significant. Homosexual or bisexual men, regardless of whether they were also intravenous drug users, had substantially higher risks of Kaposi's sarcoma than any other group. People born in Caribbean and African countries whose HIV was acquired by heterosexual contact had the next highest risk, with 6% having Kaposi's sarcoma. Men with haemophilia and women who acquired HIV by heterosexual contact had the lowest risks. The percentage of Kaposi's sarcoma varied by race within each transmission group, but the differences between races were not consistent. Among homosexual or bisexual men, the percentage with Kaposi's sarcoma was twice as high in whites as in blacks. In contrast, among transfusion recipients, the percentage of whites with Kaposi's sarcoma was half that for blacks. Among intravenous drug users, there was a slight and nonsignificant excess among whites compared with blacks. The percentage of Kaposi's sarcoma increased with age up to 35–44 years in homosexual or bisexual men, then declined. The shape of the age curve in these men has changed over time, with the peak moving to progressively older ages. Among adults in other risk groups, there was little variation in the percentage with Kaposi's sarcoma by age. Kaposi's sarcoma is rare before the age of 15 years. The proportion of patients with Kaposi's sarcoma was 1.6% for those aged less