OBJECTIVE: Recent studies have demonstrated beneficial metabolic effects of dietary monounsaturated fatty acids (MUFA) in Type-2 diabetes mellitus (Type-2 DM). The question arises if dietary MUFA also has desirable effects on risk markers in subjects with high risk of developing Type-2 DM. SETTING: University department of endocrinology. SUBJECTS: Sixteen healthy, first-degree relatives (six men, 10 women, age (mean+/-s.d.): 35+/-2 years) with normal oral glucose tolerance tests. INTERVENTIONS: Randomised study with two 4-week treatment periods with either a carbohydrate-rich (CHO) diet (55 E% carbohydrate, 30 E% fat, 15 E% protein) or a diet rich in olive oil [MUFA 40 E% fat (25 E% as MUFA), 45 E% carbohydrate, 15 E% protein]. The periods were divided by a 4-week wash-out period. RESULTS: Similar lowering effects on total cholesterol, low density lipoprotein (LDL)-cholesterol, triglyceride and apoB levels were seen after the two diets. Slightly higher levels of high-density lipoprotein (HDL)-cholesterol (1.4+/-0.4 vs 1.3+/-0.4 mmol/l, P<0. 0001) and apoA-1 (1.2+/-0.3 vs 1.1+/-0.3 mmol/l, P<0.05) were found in the MUFA-diet. Furthermore, the insulin sensitivity, as assessed by Bergman's minimal model, and the first response insulin areas were similar, as were the 24-h blood pressures and the von Willebrand Factor (vWF) levels. CONCLUSIONS: Isocaloric diets rich in MUFA or rich in carbohydrate, respectively, seem to have similar effects on cardiovascular risk factors in persons at high risk of developing Type-2 DM. A potential risk, however, on body weight of high-fat diets should be kept in mind. SPONSORSHIP: This study was supported by grants from the Danish Diabetes Association, Institute of Clinical Experimental Research, Aarhus University, The Danish Heart Foundation, The Danish Medical Research Council, Velux Foundation, Poul and Erna Sehested Hansens Foundation, Mogens Svarre Mogensens Foundation.