A familial erythrocytosis is described. The propositus is a 52-year-old man who was found, on routine testing, to have an erythrocytosis with a Hb level of 19.8 g/dl. Physical examination and laboratory findings revealed an associated hemolysis. The P50 of the stripped hemolysis was normal. The 2,3-DPG level was reduced to 15% of normal but that of ATP was increased more than twofold. Red cell diphosphoglycerate mutase activity was normal as were the levels of all red cell glycolytic and hexose monophosphate shunt enzymes with the exception of pyruvate kinase (PK). The level of the latter was elevated to 24.5 U/g (normal: 6.2 +/- 1.1). The electrophoretic pattern of hemolysate PK was abnormal, showing three additional bands, one of these migrated as the M2 isozyme. The findings were similar in the propositus's daughter and her daughter and in one of the two brothers of the propositus. The second brother and the son of the first brother was exhibited erythrocytosis but their red cell 2,3-DPG levels were normal. In addition, the level of their RBC PK was reduced to between 50% and 60% of normal and the abnormal electrophoretic bands were absent. The erythrocytosis appears to be inherited as an autosomal-dominant trait. The relationship between the PK abnormalities appears complex.