The relations between the incidence of vascular occlusive episodes and packed-cell volume (P.C.V) and platelet-count in patients with primary proliferative polycythaemia were determined retrospectively in patients treated by venesection with or without chemotherapy. The incidence of occlusive episodes correlated positively with P.C.V. level. The risk of vascular occlusive episodes was increased at moderately increased P.C.V. levels and the optimum P.C.V. level was rather lower than is often assumed. There was no statistically significant association between platelet-count, either alone or in combination with raised P.C.V., and incidence of vascular occlusion, though, episodes of occlusion were 1.5 times more common with platelet-counts above 400 x 10(9)/l. The results of this study indicate that P.C.V. should be maintained at less than 0.45 and the platelet-count at less than 400 x 10(9)/l in primary proliferative polycythaemia.