Previous studies have shown that unsaturated fat-enriched diets may have a beneficial effect on blood pressure in non-insulin-dependent diabetic (NIDDM) patients, whereas little is known about the effects on albuminuria. In a 3-week cross-over design we compared the effects of a currently recommended high-carbohydrate diet (50% carbohydrate, 30% fat [10% monounsaturated fat]) vs a diet rich in monounsaturated fat (30% carbohydrate, 50% fat [30% monounsaturated fat]) on urinary albumin excretion rate, 24-h ambulatory blood pressure and metabolic control in ten NIDDM patients with persistent microalbuminuria. The 24-h ambulatory blood pressure was similar before and after both the high-carbohydrate diet (mean +/- SD: 145/78 +/- 25/10 vs 143/79 +/- 19/10 mmHg (NS) and the monounsaturated fat diet: 140/78 +/- 16/8 vs 143/79 +/- 15/8 mmHg (NS). No changes were observed in day or night-time blood pressures. Urinary albumin excretion rate was unaffected after 3 weeks' treatment by the diets: from (geometric mean x/divided by tolerance factor) 32.4 x/divided by 2.1 to 36.0 x/divided by 1.9 micrograms/min (NS) vs from 34.2 x/divided by 1.9 to 32.1 x/divided by 2.1 micrograms/min (NS). Fasting plasma glucose, serum fructosamine and HbA1c as well as lipid and lipoprotein concentrations were stable during both diets. Compared to the high-carbohydrate diet a reduction in the LDL/HDL cholesterol ratio was observed during the monounsaturated fat diet (p < 0.03). In conclusion, compared to a high-carbohydrate diet, 3 weeks' treatment with a monounsaturated fat diet did not affect the levels of 24-h ambulatory blood pressure or albuminuria in microalbuminuric NIDDM patients.(ABSTRACT TRUNCATED AT 250 WORDS)