The aggregation responses of platelets from 19 patients who had completed treatment of acute lymphocytic leukemia (ALL) and had remained in remission for two months to 4.8 years were studied both in vitro and in vivo. Responses to 0.5 micro M adenosine diphosphate (ADP), epinephrine, and collagen were all normal. One child had irreversible platelet aggregation to 0.1 micro M ADP, a concentration that only elicited a primary aggregation response from control platelets. The mean platelet-aggregate ratio was normal. We conclude that platelet defects, although a possible late effect of ALL or its chemotherapy, do not occur with any appreciable frequency in long-term survivors.