Glomerular filtration rate (GFR), renal plasma flow (RPF), kidney volume, and urinary albumin excretion rate were measured in 24 insulin-dependent diabetics, aged 29 +/- 7 years (mean +/- S.D.) of 8 +/- 4 years duration, randomly allocated to either continuous subcutaneous insulin infusion (CSII) (n = 12) or unchanged conventional insulin treatment (CIT) (n = 12). Glomerular filtration rate, renal plasma flow, and kidney volume were identical in the two groups at the start of the study, although significantly increased above normal values. During the 6 months CSII treatment a reduction of the GFR from 145 +/- 21 to 132 +/- 14 ml/min (2p = 2.4%) was seen, no change was observed in the CIT group while in both groups RPF and kidney volume remained unchanged. Urinary albumin excretion rate was normal or near normal in both groups and remained unchanged. Thus improved glycaemic control in insulin-dependent diabetics studied before the onset of microalbuminuria is associated with improved (reduced) GFR. Nephromegaly does not improve with 6 months CSII treatment. Whether it would improve with more prolonged treatment is uncertain.