Glomerular and tubular microproteinuria precede the development of overt nephropathy in Type 1 diabetes mellitus. However, in Type 2 diabetes urinary protein excretion and its relationship to diabetic nephropathy has not been clearly characterized. Twenty consecutive, newly diagnosed patients with Type 2 diabetes, whose urine was Albustix-negative and sterile on culture, were studied. Two timed overnight urine samples were collected at diagnosis, and after 2 months and 2 years, and excretion rates of albumin, alpha-1-microglobulin and N-acetyl-beta-D-glucosaminidase were calculated. HbA1c fell from 12.1 +/- 2.4% at diagnosis to 9.5 +/- 1.5% at 2 months and 9.6 +/- 2.2% at 2 years. Albumin excretion rate fell marginally from 6.5 (2.1-242.5) micrograms min-1 at diagnosis to 5.5 (1.7-274.0) micrograms min-1 at 2 months (p less than 0.05) rising again to 6.1 (1.9-201.7) micrograms min-1 at 2 years. alpha-1-Microglobulin excretion rate fell from 13.5 (3.6-59.9) micrograms min-1 at diagnosis to 8.4 (2.9-16.1) micrograms min-1 at 2 months and 8.8 (1.8-54.1) micrograms min-1 at 2 years (both p less than 0.05). Albumin excretion rate was found to correlate significantly with creatinine clearance at diagnosis (rs = 0.61, p less than 0.005), though not subsequently. In contrast, excretion rates of alpha-1-microglobulin and N-acetyl-beta-D-glucosaminidase correlated with HbA1c (rs = 0.68 and 0.66, respectively, p less than 0.005 at diagnosis and rs = 0.57 and 0.53, p less than 0.05 subsequently in both cases).(ABSTRACT TRUNCATED AT 250 WORDS)