Urinary iron excretion rate(u-FeER, microgram/min) in urine at night time from 58 patients with non-insulin dependent diabetes mellitus and 8 controls was measured by atomic absorption. The patients were divided into three groups according to urinary albumin excretion rate(u-AER), namely, Group I(n = 44): less than 20 micrograms/min of u-AER; Group II(n = 9): 20 < or = u-AER < 200 micrograms/min; Group III(n = 5): more than 200 micrograms/min of u-AER. The u-FeER in group III(56.3 +/- 14.8 ng/min, mean +/- SEM) was significantly higher than that in controls (4.0 +/- 1.6ng/min), group I(8.3 +/- 1.6 ng/min) and group II(18.5 +/- 6.5ng/min). The increase of u-FeER in group III indicates that urinary iron may at least partly play a role in development of diabetic nephropathy.