We established the diagnostic criteria of pure erythrocytosis (PE) and spurious polycythemia (SP), and investigated the different points between 4 PE patients and 4 SP patients who had clinical course longer than 5 years from diagnosis. The red cell mass (RCM) was greater in PE patients than in SP patients, but the difference was not significant. However, the actual measurement of RCM was significantly greater in PE than their predicted values (p less than 0.01), and was not significant in SP. Such laboratory data as red cell counts, Hb, Hct, reticulocyte counts, M/E ratio of bone marrow pictures, the positive rate and score of NAP, and bone marrow CFU-E numbers were not significantly different in the two groups, and serum EPO levels in patients with PE were significantly lower than those in patients with SP (p less than 0.01). When in hospital, the Hct levels of patients of both groups decreased linearly. Sustained erythrocytosis was found in two patients with PE and one patient with SP. Vascular complications in patients of both groups have not been observed. There was no essential difference between the two groups. Therefore, it is suggested that PE patients without endogenous erythroid colonies may follow almost the same clinical course as SP patients.