Reduction of excessive haematocrit levels in patients with polycythaemia due to hypoxic lung disease by phenylhydrazine hydrochloride and pyrimethamine.

A series of fourteen patients with polycythaemia due to hypoxic lung disease has been treated with a combination of phenylhydrazine hydrochloride and pyrimethamine to reduce the haematocrit level, with doses usually of up to 100 mg/day (and rarely up to 150 mg/day) of the former and never more than 12·5 mg/day of the latter. Successful reduction of the haematocrit level was achieved in every case without any significant thrombocytopenia, the lowest level ever occurring being 100,000 platelets/mm in two patients. All the patients were improved symptomatically.