Circulating immune complexes (ClC) were detected by platelet aggregation test (PAT) in 40.0% of 45 diabetics and by polyethylene glycol precipitation-complement consumption test (PEG-CC test) in 30.6% of 36 diabetics as compared to 5% and 10% of 20 normal control subjects for each test. The prevalence of CIC in diabetics was significantly higher than in the normal controls (P<0.05%). There were no correlations between the presence of ClC detected by PAT and the duration of the disease, insulin treatment, or diabetic complications. Thus multiple factors must contribute to the increase of ClC in diabetics. The role of these various factors needs to be studied.