The study investigates the accumulation of miR-155 and BIC RNA in human B cell lymphomas. MiR-155, a microRNA encoded within the non-coding RNA BIC RNA, is found to be processed from sequences present in BIC RNA, which accumulates in lymphoma cells. The precursor of miR-155 is likely a transient spliced or unspliced nuclear BIC transcript rather than accumulated BIC RNA, which is primarily cytoplasmic. Clinical isolates of several types of B cell lymphomas, including diffuse large B cell lymphoma (DLBCL), show 10- to 30-fold higher copy numbers of miR-155 than normal circulating B cells. Similarly, the quantities of BIC RNA are elevated in lymphoma cells, but the ratios of the amounts of the two RNAs are not constant, suggesting that the level of miR-155 is controlled by transcription and processing. Significantly higher levels of miR-155 are present in DLBCLs with an activated B cell phenotype than with the germinal center phenotype, which has poorer clinical prognosis. The study suggests that quantification of miR-155 may be a useful diagnostic tool for B cell lymphomas.The study investigates the accumulation of miR-155 and BIC RNA in human B cell lymphomas. MiR-155, a microRNA encoded within the non-coding RNA BIC RNA, is found to be processed from sequences present in BIC RNA, which accumulates in lymphoma cells. The precursor of miR-155 is likely a transient spliced or unspliced nuclear BIC transcript rather than accumulated BIC RNA, which is primarily cytoplasmic. Clinical isolates of several types of B cell lymphomas, including diffuse large B cell lymphoma (DLBCL), show 10- to 30-fold higher copy numbers of miR-155 than normal circulating B cells. Similarly, the quantities of BIC RNA are elevated in lymphoma cells, but the ratios of the amounts of the two RNAs are not constant, suggesting that the level of miR-155 is controlled by transcription and processing. Significantly higher levels of miR-155 are present in DLBCLs with an activated B cell phenotype than with the germinal center phenotype, which has poorer clinical prognosis. The study suggests that quantification of miR-155 may be a useful diagnostic tool for B cell lymphomas.