Two siblings, with a congenital hemorrhagic state characteristic of thrombasthenia, had a history of frequent ecchymoses and prolonged bleeding following minor abrasions. Results of laboratory studies revealed normal platelet counts, long bleeding times, deficient platelet factor 3 availability, and absence of platelet aggregation following the addition of adenosine diphosphate, collagen, and epinephrine. Results of other clotting studies were normal. Three bleeding episodes, two gastrointestinal hemorrhages and one subdural hematoma, were treated with platelet concentrates with clinical and laboratory evidence of restoration of the platelet mediated phase of hemostasis. Platelet infusions appear to be effective in treatment of serious bleeding in these patients.