Plasma concentrations of 6-keto-PGF1 alpha and thromboxane B2 (TXB2) were measured in 55 diabetics and 30 controls with radioimmunoassay. The correlation between diabetic microangiopathy and the ratio of TXB2 and 6-keto-PGF1 alpha was analysed. Diabetics were divided into three groups according to the change of retina and renal function. Group A, diabetics without microangiopathy; group B, diabetics with slight microangiopathy; and group C, diabetics with severe microangiopathy. The results showed that the ratio of TXB2/6-keto-PGF1 alpha was higher in B and C groups (0.997 +/- 0.31 1.10 +/- 0.25 means +/- S) than in the controls (0.72 +/- 0.17 means +/- S) (P less than 0.01). Group A patients had slightly higher level of the ratio (0.85 +/- 0.20 means +/- S) than the controls but the difference was not significant. The results suggest that the TXB2 and 6-keto-PGF1 alpha imbalance exists only in diabetics with microangiopathy and their imbalance might have more significance in the pathogenesis of diabetic microangiopathy than their individual change.