This study was designed to assess the density characteristics of platelets from controls (N = 10) and three groups of diabetics (N = 32) exhibiting various degrees of glycemic control. With continuous gradients of Percoll, platelets from controls and diabetics (N = 8) with an HbA1 less than or equal to 9% formed a band extending from 1.0625 g/ml to 1.0925 g/ml with a mean platelet density of 1.0775 g/ml. In the two groups of diabetics with HbA1 greater than or equal to 10%, there was an increase in the proportion of low-density platelets recovered on the gradients and the mean platelet density was reduced to 1.0750 g/ml (HbA1 = 10-13%) and 1.070 g/ml (HbA1 greater than or equal to 14%). All three groups of diabetics had normal levels of intraplatelet ATP/ADP and beta-thromboglobulin. It is unlikely that in vivo degranulation of platelets after activation was responsible for the altered density profiles. We propose that abnormal platelet subpopulations with low density but normal intraplatelet granule content were responsible for the changed density profiles.