As part of an epidemiological survey of myelodysplastic syndromes (MDS) in southern Tasmania, 62 MDS patients identified over a 2 year period were tested for the presence of CD34, the human progenitor cell antigen (HPCA), in their peripheral blood. The results were correlated with transformation to acute myeloid leukemia (AML) and patient survival, and CD34+ status was compared as a prognostic indicator with Bournemouth score, cytogenetics, and CFU-GM colony growth which were also assessed. Circulating CD34+ cells were found in 23 of the 62 MDS patients; 9 of the 23 patients with circulating CD34+ cells transformed to AML, as compared with none of the 39 CD34 negative patients (P less than 0.0001); and 11 of the 23 patients with circulating CD34+ cells were dead at the end of the 2 year period, as opposed to 6 of the 39 with no CD34+ cells (P less than 0.03). The Bournemouth score was also significantly associated with transformation to AML (P less than 0.0001) and poor survival (P less than 0.04). These were the only significant associations of the possible prognostic factors studied with either transformation or survival. In summary, the presence of circulating CD34+ cells was significantly associated with both progression to AML and poor survival and was found to be a better prognostic indicator than cytogenetics or CFU-GM colony growth.