The aim of the present study was to evaluate the role of the strict metabolic control achieved with the aid of an artificial pancreas (Biostator, Miles) on platelet functions in insulin-dependent diabetics (n = 20). Platelet aggregation responses to both ADP and collagen, as well as circulating platelet aggregates according to Wu and Hoak, were determined before and after 24 hours of complete metabolic control; as control group, 18 insulin-dependent diabetics were only monitored without attempts to normalize their blood glucose behaviour. A significant decrease in ADP induced aggregation (p less than 0.01) and circulating platelet aggregates (p less than 0.001) was observed in diabetics during the period of strict metabolic control. No changes were observed in the control group. The present data suggest that the increased platelet activity in diabetes is a consequence of the metabolic derangement of the disease.