Albumin excretion rate, glomerular filtration rate (GFR) and kidney volume in obese patients and normal-weight controls were compared with body mass index (weight (kg)/height2 (m)) in 17 subjects. Body mass index varied from 21.5 to 48.0, the albumin excretion rate from 2.8 to 17.8 micrograms/min, and the kidney volume from 238 to 468 ml. Body mass index correlated significantly with albumin excretion rate and with kidney volume (p < 0.01), but not with the GFR. Neither the body mass index nor the albumin excretion rate showed any correlation with blood pressure. Albumin excretion rate in obese subjects could be as good an early predictor of complications as it is in patients with diabetes mellitus, and in the elderly.