In 14 female type I diabetics the influence of preexisting diabetic nephropathy on the behaviour of proteinuria and renal function during and after pregnancy was investigated. We compared albumin and total protein in urine, serum-creatinine, creatinine-clearance, uric acid and beta 2-microglobulin in serum before, during and up to sixth months after pregnancy in 5 diabetic women with preexisting normo-albuminuria, stage II of diabetic nephropathy, in 6 women with microalbuminuria, stage III, and in 3 women with overt proteinuria, stage IV of diabetic nephropathy (by Mogensen); in this last patient-group there was a heavy proteinuria with decreased creatinine clearance and high blood pressure still before the begin of the pregnancy. The albumin- and total protein excretion in urine showed in all diabetic patients a 3- to 8-fold increase during pregnancy and in all cases the increased proteinuria dropped after pregnancy to the preexisting values. A transient nephrotic syndrome in pregnancy developed in 2/6 diabetic women with primary microalbuminuria and in all 3 patients with overt proteinuria, but not in any case of pregnant women with primary normo-albuminuria; the difference between the absolute increase of proteinuria in the 2 patient-groups with stage II and III of diabetic nephropathy was statistically significant (p less than 0.005). The renal function parameters serum-creatinine and creatinine-clearance showed in all pregnant women with stage II or III of diabetic nephropathy the same transient changes as in non-diabetic pregnant women, the mean serum-creatinine concentration showed a decline of 36% respectively 14%, the creatinine clearance an increase of 31% and 36%.(ABSTRACT TRUNCATED AT 250 WORDS)