In the past ten years, almost every mode of inheritance, including sex-linked dominant, has been proposed for the Noonan syndrome. The occurrence of the phenotype in father and son reported in this and two other communications makes X-linked dominant hypothesis untenable. It appears that the inheritance of the Noonan syndrome could best be explained by an autosomal dominant gene with variable expressivity. The proband, in the present communication, had truncus arteriosis type 4, not previously described in this syndrome.