We report seven children with a clinically benign form of primary splenomegaly associated with hematologic evidence of hypersplenism. Five belong to one sibship, the other two are second cousins of each other and of the group of siblings. Splenectomy was performed in five subjects, but pedigree evidence of this autosomal dominant trait tends to indicate that complete clinical resolution occurs normally so that transmitting adults show no splenomegaly or hypersplenism. This appears to be a newly recognized Mende‐lian mutation which is evident (“penetrant”) only in childhood, and which may possibly have pleiotropic effects on eye and CNS development.