To the Editor. —There is more evidence that genetic factors predispose to chronic immunologic thrombocytopenic purpura (ITP) than is summarized by Bogart and Wittels in their article on ITP in two elderly siblings in the December 1985 issue of theArchives. 1 Among 22 patients with chronic ITP seen in our clinic, three patients had a relative with the disorder. 2 The family of one of the probands was studied in detail. 3 Only two members of the kindred had ITP, but many others had diverse autoimmune diseases, serologic abnormalities, or both. 3 In another kindred, three siblings, including monozygotic twins, had chronic ITP. 4 Autoimmune diseases including ITP are not inherited, but there is abundant evidence that in numerous cases there is a genetically determined predisposition. Many patients with chronic ITP sooner or later have had seemingly unrelated autoimmune disorders, notably thyroid disease, autoimmune hemolytic anemia, or polyarthritis. Onset