The subjects of this study were 54 hospitalized men whose clinical evaluation included a glucose tolerance test. Platelet aggregation induced by 1.7 μM ADP at room temperature was measured turbidimetrically in platelet‐rich plasma (PRP) derived from each man's fasting blood sample. The men were classified as to whether they had ischaemic heart disease or diabetes mellitus (2‐h glucose, 140 mg/100 ml or more) and by age and race. No significant difference between mean platelet aggregation values was found for any of these 4 comparisons. Platelet counts were done with 31 PRP, and nonesterified fatty acid (NEFA) and triglyceride concentrations were determined in 21 of the fasting plasmas. There was no significant correlation between platelet aggregation and age or between platelet aggregation and the plasma concentrations of glucose, platelets, triglycerides, total nonesterified fatty acids (NEFA) or any of the 6 major individual NEFA.