We have measured skin mast cell numbers, circulating basophils and whole blood histamine in 13 patients with polycythaemia vera. Itching was present in nine cases and correlated well with the numbers of skin mast cells but not with circulating basophils or whole blood histamine. Immediate relief of pruritus was achieved with aspirin, and myelosuppressive therapy was useful for long-term control of symptoms. Neither histamine (H1 or H2) antagonists nor iron replacement therapy were effective forms of treatment. The findings suggest that mast cell prostaglandins are an important factor in the pathogenesis of pruritus and that local vascular responses may trigger mast cell degranulation.