The effect of gemfibrozil, a new hypolipidaemic compound, on serum lipids, lipoproteins and ophthalmic signs was studied in 20 diabetics during a 28 week trial. Ten of the patients were taking oral hypoglycaemic agents and another ten insulin. Genfibrozil elevated the proportion of alpha-lipoproteins (HDL) (P less than 0.005) in patients on oral agents to the same level as in those using insulin, who experienced no change in HDL. The pre-beta-lipoprotein proportion (VLDL) decreased (P less than 0.001) along with the serum triglyceride level in patients on oral agents only. The proportion of beta-lipoproteins (LDL) remained unchanged in both groups. The serum cholesterol and total lipid levels decreased significantly whether the patients were on oral treatment or insulin. The gemfibrozil was well tolerated. A minor increase in diabetic therapy was made in 9 patients in order to control the blood sugar levels. The ophthalmic signs were generally unchanged.