Familial erythrocytosis.

Erythrocytosis was found in 3 generations of an English family. The red cell mass was only moderately increased in some of the affected members but was accompanied by an unusually low plasma volume. Oxygen affinity of Hb was normal. Changes in serum and urinary erythropoietin showed an essentially normal pattern throughout a series of venesections. Venesection produced some transient clinical benefit in the older patient but there was no obvious change in the younger ones.