Within the same family erythrocytosis occurred in the mother, in three of five children and possibly in one child of an affected daughter. The occurrence of the anomaly in successive generations as well as the distribution of affected and healthy persons in the offspring of affected individuals are in excellent agreement with autosomal dominant inheritance.Hemoglobin function (P50) and red cell function (2,3‐diphosphoglycerate) were normal. Plasma erythropoitin activity in plasma of affected individuals was undetectable when measured by cell culture assay. Only low levels of activity, comparable to that occurring in normal plasma, were detected using59Fe incorporation in erythrocytes of exhypoxic, polycythemic mice and this activity did not increase upon phlebotomy of 15–20% of the blood volume. Thus, erythrocytosis was not caused by increased erythropoitin production but some other, unknown mechan