Dysregulated expression of the c-myc and c-myb protooncogenes has been implicated in the pathogenesis of acute myeloid leukemia (AML). To elucidate mechanisms of c-myc dysregulation in AML cells, we studied c-myc RNA turnover in peripheral blood blasts from eight patients using actinomycin D transcription blockade. Rapid c-myc RNA turnover was seen in cells from six patients, with half-lives of approximately 30 minutes, similar to those reported in normal myeloid cells, in HL-60 cells, and in other cell lines. c-myc RNA turnover was prolonged in cells of the other two patients, with half-lives of greater than 75 minutes. c-fos RNA turnover was rapid in blasts from all eight patients, with half-lives of approximately 15 minutes. Stabilization of GM-CSF transcripts was not observed. In contrast, c-myb RNA half-lives were greater than 75 minutes in cells of the two patients with prolonged c-myc RNA turnover, as compared to 30 minutes in cells of the other six patients. Enhanced stability of both c-myc and c-myb RNA species suggests that a defect exists in a trans-acting factor that destabilizes both of these normally labile RNAs. Incomplete correlation between c-myc RNA levels and half-lives indicates regulation of c-myc expression at the level of transcription or nuclear transport in addition to posttranscriptional regulation.