Deficient leucocyte sphingomyelinase activity has been demonstrated in a patient with the sea-blue histiocyte syndrome. Family studies revealed that two other cases previously diagnosed on clinical and histochemical criteria also had a pronounced diminution of sphingomyelinase activity. Both parents of the affected individuals were carriers of the disease as indicated by sphingomyelinase activity intermediate between normal and diseased subjects. Additional heteroxygous carriers were found among the siblings and other relatives of the patients. This family study supports further the hypothesis that the sea-blue histiocyte syndrome and chronic Niemann-Pick (Type B) disease are the same.