Urinary protein excretion rate and clinical and metabolic associates were investigated in a group of 108 patients with Type 2 diabetes mellitus at the time of diagnosis and after 5 years, and also 121 control subjects. The presence of coronary heart disease, neuropathy and retinopathy, cardiovascular risk factors and 24-h urinary excretion rate of albumin, beta-2-microglobulin, and IgG were examined. At the 5-year examination, urinary excretion rate of albumin was higher in diabetic patients than in control subjects (39 +/- 75 vs 16 +/- 28 mg 24 h-1 for men, p < 0.05; 38 +/- 57 vs 22 +/- 42 mg 24(-1) h for women, p < 0.01). Furthermore, increased beta-2-microglobulin excretion rate, a marker of tubular impairment, was observed in diabetic men as compared to control men (0.17 +/- 0.15 vs 0.14 +/- 0.21 mg 24 h-1, p < 0.05). Diabetic patients with increased albumin excretion rate (> 30 mg 24 h-1) showed poorer metabolic control than those with normal albumin excretion rate, but no significant differences in body mass index or in the frequencies of smoking, hypertension, coronary heart disease or retinopathy and neuropathy were observed between the groups. Baseline hyperinsulinaemia was closely associated with increasing albuminuria at the 5-year examination.