Microalbuminuria has recently been associated with insulin resistance in both insulin-dependent and non-insulin-dependent (NIDDM) diabetes mellitus. To establish whether microalbuminuria in non-diabetic subjects as well is associated with insulin resistance and associated abnormalities in glucose and lipid metabolism, oral glucose tolerance tests were performed with measurement of urinary albumin excretion rate, lipids and lipoproteins in 582 male non-diabetic first-degree relatives of patients with NIDDM. In addition, insulin sensitivity was assessed in 20 of these subjects with the euglycaemic hyperinsulinaemic clamp technique. Abnormal albumin excretion rate (AER), defined as AER 15-200 micrograms/min, was associated with higher systolic blood pressure (p < 0.05), higher fasting glucose values (p < 0.05), lower HDL-cholesterol (p < 0.05) and lower apolipoprotein A-I (p < 0.05) concentrations than observed in subjects with normal AER. The rate of glucose metabolism was lower in subjects with abnormal compared to subjects with normal albumin excretion rate (38.0 +/- 2.8 vs 47.3 +/- 2.4 mumol.kg lean body mass-1.min-1; p = 0.028). This difference was almost completely accounted for by a reduction in non-oxidative glucose metabolism (17.7 +/- 1.9 vs 27.4 +/- 2.7 mumol.kg lean body mass-1.min-1; p = 0.010), which correlated inversely with the AER (r = -0.543; p = 0.013). These results suggest that in non-diabetic individuals genetically predisposed to NIDDM, abnormal AER is associated with insulin resistance and abnormalities in glucose and lipid metabolism.