This study aimed to evaluate infection characteristics, incidence, and risk factors associated with acute pyelonephritis in a sample of healthy women. The study was a population-based case-control study conducted at Group Health Cooperative in Washington. Participants included 788 nonpregnant women aged 18 to 49 years, with 242 cases of pyelonephritis and 546 controls without a recent diagnosis of pyelonephritis. The main findings were: - **Incidence and Infection Characteristics**: The estimated annual incidence of pyelonephritis was 27.6 cases per 10,000 women. - **Causative Organism**: *Escherichia coli* was the predominant infecting organism in 85% of cases. - **Risk Factors**: Factors associated with an increased risk of pyelonephritis included: - Frequent sexual intercourse in the previous 30 days (odds ratio, 5.6 [95% CI, 2.8 to 11.0]). - Recent urinary tract infection (UTI) (odds ratio, 4.4 [CI, 2.8 to 7.1]). - Diabetes (odds ratio, 4.1 [CI, 1.6 to 10.9]). - Recent incontinence (odds ratio, 3.9 [CI, 2.6 to 5.9]). - New sexual partner in the previous year (odds ratio, 2.2 [CI, 1.4 to 3.6]). - Recent spermicide use (odds ratio, 1.7 [CI, 1.1 to 2.8]). - UTI history in the participant’s mother (odds ratio, 1.6 [CI, 1.1 to 2.5]). - **Subgroup Analysis**: The risk factors were also evaluated in selected subgroups, including women ≤30 years, women >30 years, women with no UTI history, and inpatients. - **Discussion**: The findings suggest that sexual behaviors and patient and family history of UTI are associated with increased risk of pyelonephritis. Diabetes and incontinence also independently increase the risk. The study highlights the importance of these risk factors in the epidemiology of acute pyelonephritis in healthy women.This study aimed to evaluate infection characteristics, incidence, and risk factors associated with acute pyelonephritis in a sample of healthy women. The study was a population-based case-control study conducted at Group Health Cooperative in Washington. Participants included 788 nonpregnant women aged 18 to 49 years, with 242 cases of pyelonephritis and 546 controls without a recent diagnosis of pyelonephritis. The main findings were: - **Incidence and Infection Characteristics**: The estimated annual incidence of pyelonephritis was 27.6 cases per 10,000 women. - **Causative Organism**: *Escherichia coli* was the predominant infecting organism in 85% of cases. - **Risk Factors**: Factors associated with an increased risk of pyelonephritis included: - Frequent sexual intercourse in the previous 30 days (odds ratio, 5.6 [95% CI, 2.8 to 11.0]). - Recent urinary tract infection (UTI) (odds ratio, 4.4 [CI, 2.8 to 7.1]). - Diabetes (odds ratio, 4.1 [CI, 1.6 to 10.9]). - Recent incontinence (odds ratio, 3.9 [CI, 2.6 to 5.9]). - New sexual partner in the previous year (odds ratio, 2.2 [CI, 1.4 to 3.6]). - Recent spermicide use (odds ratio, 1.7 [CI, 1.1 to 2.8]). - UTI history in the participant’s mother (odds ratio, 1.6 [CI, 1.1 to 2.5]). - **Subgroup Analysis**: The risk factors were also evaluated in selected subgroups, including women ≤30 years, women >30 years, women with no UTI history, and inpatients. - **Discussion**: The findings suggest that sexual behaviors and patient and family history of UTI are associated with increased risk of pyelonephritis. Diabetes and incontinence also independently increase the risk. The study highlights the importance of these risk factors in the epidemiology of acute pyelonephritis in healthy women.