Glomerular filtration and effective renal plasma flow were studied in 24 patients with diabetes mellitus type I (mean age 29.8 +/- 10.5 years) and 20 patients with diabetes mellitus type II (mean age 54 +/- 11 years) and duration of diabetes 5.7 +/- 6 and 11.3 +/- 10 years respectively. There were no clinical signs of nephropathy in all patients studied (the albustick test was negative). The results were compared with those of 30 healthy controls. Out of the 24 patients with diabetes mellitus type I 10 (41%) were with hyperfiltration and from the 20 patients with diabetes mellitus type II only one was with hyperfiltration. The speed of albumin secretion in the diabetic patients with hyperfiltration was 39.9 +/- 42.9 micrograms/min and was significantly higher than that of the diabetic patients without hyperfiltration--26.2 +/- 22.7 micrograms/min. The albuminuria was in a moderate positive correlation with the glomerular filtration. The possible mechanism taking part in the development of hyperfiltration syndrome in diabetes are discussed.