Sixty-six adults with chronic autoimmune thrombocytopenic purpura AITP and platelet count <50 x 10(9)/l were treated with dapsone (75-100 mg orally). A response was observed in 33 patients. The median duration of treatment required to obtain a response was 21 d (range 8-90). The median maximal platelet count on treatment was 130 x 10(9)/l (range 71-355). Dapsone was continued in 20/33 responders for a median of 12.5 months (range 1-48) and the response persisted in 19. Treatment was intentionally withdrawn in the other 13 responders and thrombocytopenia immediately recurred in 12. Reversible side-effects required cessation of treatment in seven patients. These results demonstrate that dapsone is an effective, inexpensive, and well-tolerated treatment for chronic AITP.