Among 59 consecutive patients with myelofibrosis (MF) in whom peripheral blood (PB) cytogenetic studies were performed, at least two analyzable metaphases (median 20, range 2-31) were obtained in 49 (81%) patients and in all 37 (100%) cases with PB myeloid progenitor cell count of 0.1 x 10(9)L(-1) or above (p=0.02). Twenty-two patients had concomitant PB and bone marrow (BM) cytogenetic studies; 6 showed similarly abnormal findings in both BM and PB. In another 2 cases, results were abnormal in BM but normal in PB; the opposite was seen in 1 case. These results suggest that PB can be considered as an alternative to BM for cytogenetic studies as currently used in MF but additional prospective studies are needed to support change in practice.