This study was designed to investigate the importance of risk factors such as hyperglycemia and elevated systolic and diastolic blood pressures on the progression of renal insufficiency in diabetics suffering from diabetic nephropathy. Seventeen patients with Type I, insulin-dependent diabetes mellitus (IDDM) (8 women and 9 men) undergoing chronic hemodialysis were investigated by retrospective follow-up and compared with 17 age and sex matched IDDM patients without diabetic nephropathy (controls). According to the time interval of creatinine increase from 200 to 600 mumol/l, the patients were divided arbitrarily into two groups with rapidly (group I less than 20 months) or slowly progressive (group II greater than or equal to 20 months) renal insufficiency. This period was 13.4 +/- 2.05 months in group I (age 36.67 +/- 2.47 years, diabetes duration 23.55 +/- 2.37 years) and 32.75 +/- 4.34 months in group II (age 40.62 +/- 2.63 years, diabetes duration 26.62 +/- 2.63 years, P.n.s.), respectively. The IDDM patients studied exhibited individually differing progressions of renal insufficiency at different times after manifestation of diabetes. After 15 years of diabetes duration, both risk factors, that is blood pressure and blood glucose concentrations, were elevated in nephropathic diabetics when compared with controls (p less than 0.01). During the phase of declining kidney function, mean blood pressures were found to be higher in IDDM patients with rapid progression of renal insufficiency when compared with slowly progressing diabetics. Although both risk factors were related to diabetic nephropathy, during the phase of renal insufficiency hypertension appeared to be more closely related to the further deterioration of kidney function.