Patients with primary myelofibrosis (PMF) and myelofibrosis secondary to carcinoma (SMF) were compared with regard to circulating granulocyte macrophage progenitor cells (CFU-GM) using in vitro tissue culture techniques. Although increased numbers of CFU-GM had previously been well documented in PMF, few patients with the secondary variety had been studied. Our data indicate that there is an increase in circulating CFU-GM in patients with SMF but it is significantly lower than in those with PMF. It is suggested that in both conditions disruption of the marrow microvascular system results in a release of CFU-GM to the circulation. In PMF stem cell colonization of the spleen with its consequent myeloid metaplasia may be responsible for the additional increase in CFU-GM. The determination of CFU-GM numbers may provide additional data to help to distinguish PMF and SMF in atypical cases where the distinction is unclear.