We compared urinary albumin excretion during and after pregnancy in 30 insulin-dependent diabetic (IDDM) women with normoalbuminuria and in 12 IDDM women with microalbuminuria (> 15 micrograms.min-1) prior to conception. There was a 6.7-fold increase in the urinary albumin excretion up until the third trimester in the women with pre-existing microalbuminuria, compared with a 3.8-fold increase in the normoalbuminuric women. In both groups of patients the urinary albumin excretion reached a peak during the third trimester with 492 +/- 404 micrograms.min-1 in the microalbuminuric women vs 43 +/- 36 micrograms.min-1 in the normoalbuminuric women (p < 0.0005). Two women from each of the groups developed eclampsia with diastolic blood pressure over 90 mm Hg, mild or moderate oedema and macroproteinuria. Four of the pregnant women with pre-existing microalbuminuria showed a transient nephrotic syndrome (33.3%) with protein excretion over 3 g in 24-h urine samples during the third trimester. In contrast, this was not observed in any of the normoalbuminuric women (p < 0.05). Within 12 weeks after delivery the urinary albumin excretion rates dropped to the pre-conception values in both patient groups. Renal function remained normal during pregnancy in both of the groups, with a physiological increase in creatinine clearance up until the third trimester (26% increase in the normoalbuminuric women vs 22% in the microalbuminuric women). In conclusion, the effect of pregnancy on the urinary albumin excretion in diabetic women with pre-existing microalbuminuria is an exaggeration of the increase of albuminuria in diabetic women with normoalbuminuria; normalization occurs within 12 weeks after delivery in all cases.(ABSTRACT TRUNCATED AT 250 WORDS)