Kidney volume was measured during pregnancy in insulin-dependent diabetic women by an ultrasound technique and prognostic value of these measurements evaluated. A prospective study was performed on 87 pregnant women with insulin-dependent diabetes attending the maternity clinic of Aarhus Kommunehospital. Patients with proliferative retinopathy alone, hydronephrosis, or nephrotic syndrome were excluded. The patients were grouped according to onset and duration of diabetes and to vascular lesions; group I (n = 35, White class B+C), group II (n = 11, White class D0), group III (n = 26, White class D+), and group IV (n = 15, White class F+F/R). The patients visited the hospital every 2 weeks during pregnancy for general obstetric and glycaemic control and blood sampling. The volume of both kidneys was measured by a computerized nephrosonograph during the three terms of pregnancy, the puerperium and 4 months postpartum. The kidney volume increased significantly in all four groups from first to third trimester. In the third trimester the kidney volumes were 375 +/- 68 ml (I), 341 +/- 50 ml (II), 362 +/- 63 ml (III), and 343 +/- 54 ml (IV). The kidney volume in the third trimester was positively correlated with creatinine clearance (r = 0.33, P < 0.01) and inversely correlated with creatinine in serum (r = -0.27, P = < 0.02). Total kidney volume decrease (in percent) defined as the difference of maximal volume and value at 4 months postpartum was inversely correlated to albuminuria in the third trimester (r = -0.25, P < 0.05) and vascular lesions of the patients: (mean +/- SEM) 37 +/- 4% (I), 25 +/- 7% (II), 19 +/- 5% (III), and 11 +/- 7% (IV), P < 0.01. In the puerperium, kidney volume decreased significantly from third trimester in groups I, II, and III, whereas we observed no change in group IV. Six of 15 women in groups II and III with kidney volume < 300 ml and normoalbuminuria in the first trimester developed persistent microalbuminuria after pregnancy (P < 0.02). The renal volume in insulin-dependent diabetic women increases significantly during pregnancy and is inversely related to the vascular lesions of the patients. The decrease in renal volume after pregnancy is related to the albuminuria at the end of pregnancy. Women with longstanding diabetes, White class D (= groups II+III), and kidney volume < 300 ml in the first trimester have a high risk of developing permanent microalbuminuria after pregnancy.