In this study the reliability of platelet aggregation in whole blood (WB) was investigated in clinical conditions associated with thromboembolic complications. Spontaneous (SPA) and collagen-induced platelet aggregation were evaluated both in whole and diluted blood by the impedance method and in platelet-rich plasma (PRP) by the Born aggregometer in 18 healthy subjects, 15 patients with ischemic heart disease (IHD), and 15 patients with insulin-independent diabetes. SPA occurred more often in WB than in PRP, and in WB the occurrence of SPA was significantly more frequent in the patient groups (4 of 15 patients with IHD and 6 of 15 diabetic patients) than in the controls (1 of 18). WB aggregation induced by collagen was significantly higher in patients with IHD and in diabetic patients than in controls (P less than 0.01), whereas diluted WB and PRP aggregation were not statistically different from controls either in patients with IHD or in diabetic patients. WB aggregation values were found to be related, although not very closely, to megathrombocyte count (r = 0.31, P less than 0.05) whereas not at all to platelet count or hematocrit. No relationship was observed between WB aggregation and disease severity (angiographic lesions and number of ischemic attacks) in patients with IHD and between WB aggregation and HbAlc values in diabetic patients.