The plasma factors influencing PGI2-like activity in 19 patients with diabetes mellitus (Dm) and 17 control subjects were studied by comparing the signs of retinal versus glomerular angiopathy. The plasma PGI2-supporting activity (PSA) was lower in 15 Dm cases than in the controls. Inhibitory activity against PGI2 production was detected in 6 patients. When more serious retinopathy was associated with glomerulopathy, a significantly lower level of PSA was observed than in patients with mild retinopathy without glomerular disease. The plasma concentrations of total and LDL cholesterol were significantly higher, while the level of HDL cholesterol was lower than in the controls. There was a positive correlation between PSA and HDL cholesterol values and a negative correlation between PSA and LDL cholesterol levels, which is related to the inhibitory effect of LDL and the protective role of HDL in PGI2 synthesis.