We measured simultaneous plasma beta-thromboglobulin (BTG) and adhesion of 51Cr-labelled, washed platelets to confluent, bovine aortic endothelial monolayers in 50 insulin-dependent diabetic patients and 30 normal subjects (respective mean ages (+/- SD) = 45.1 +/- 16.4 and 45.8 +/- 17.2 years). Compared to normal subjects without arteriosclerotic complications, diabetic patients had higher plasma BTG (34.8 +/- 1.8 (SEM) vs. 21.3 +/- 1.8 ng/ml) and platelet adhesiveness to endothelium (PAE) (3240 +/- 170 vs. 2430 +/- 120 X 10(3) platelets per well) (p less than 0.0002, respectively). Results in diabetic patients did not correlate with plasma glucose, hemoglobin AIa-c, known duration of disease, or sex; plasma BTG correlated with age (r = +0.36), and PAE correlated with plasma creatinine (r = +0.39). Those with clinically evident vascular disease, who were also older (47.8 +/- 2.6 (SEM) vs. 37.3 +/- 4.5 years, p less than 0.05), showed trends to higher plasma BTG (36.7 +/- 2.2 (SEM) vs. 28.8 +/- 3.4 ng/ml, p = 0.06) and PAE (3400 +/- 200 vs. 2800 +/- 280 X 10(3) platelets per well, p = 0.09). A strong correlation was found between plasma BTG and PAE in diabetic patients (r = +0.62, p less than 0.0001) either with or without vascular disease, which remained strong after statistical correction (partial Pearson correlation) for age and plasma creatinine, but not in normal subjects (r = +0.08, p greater than 0.1). These studies demonstrate that platelets in some diabetic patients are excessively adhesive to vascular endothelium, and that plasma BTG and platelet adhesiveness are intercorrelated.