Eleven insulin-dependent diabetics exhibiting a fair but less than ideal diabetic control (HbA1 = 10.0 +/- 0.6%) were submitted in a random order to two 6 week-study periods of: continuous subcutaneous insulin infusion (CSII) and optimized conventional insulin therapy. Plasma lipids, fatty acids in plasma lipids and platelet function were estimated at baseline and at the end of each study period. Declines in HbA1 were observed at the end of either CSII or conventional period compared with baseline, but the differences were only significant under CSII (P less than 0.02). Plasma lipids and apoproteins remained unchanged at the end of the two study periods compared with baseline. Both CSII and optimized conventional treatment were followed by a significant increase of arachidonate in plasma lipids. A deterioration of the platelet function estimated from ADP or epinephrine-induced platelet aggregation and TxB2 generation by platelets was found under optimized conventional treatment while the platelet function appears to be normal at baseline and under CSII. These data indicate that slight but not sufficient improvements of diabetic control can result in deterioration of the platelet function. It seems that these deleterious effects are mediated through an increased production of arachidonate and in turn of TxB2.