Nineteen patients affected by non-insulin dependent diabetes mellitus (NIDDM), in good glycemic control (fasting plasma glucose 7.2 +/- 0.3 mmol/L, glycosylated hemoglobin 6.3 +/- 0.2%), underwent three isocaloric dietary phases. In phases 1 and 3 the diet was rich in complex carbohydrates (Carbo) whereas in phase 2 it was rich in monounsaturated fatty acids (Mono). Plasma glucose concentrations were 7.1 +/- 0.3 and 7.2 +/- 0.3 mmol/L for the two Carbo phases and 7.5 +/- 0.4 mmol/L for the Mono phase (NS). Plasma total cholesterol values for the Carbo phases were 6.2 +/- 0.2 and 6.4 +/- 0.2 mmol/L, respectively, and 6.5 +/- 0.2 mmol/L on the Mono phase (NS). Similarly, no significant changes were noticed for plasma triglycerides and high-density-lipoprotein (HDL) cholesterol. Thus, both diets were well-tolerated and did not alter glucose homeostasis or worsen plasma lipid concentrations. Consequently, these results suggest that a wider dietary choice can be made available to NIDDM patients without producing unwanted side effects.