Sixteen kidney transplant (KT) patients (10 men, 6 women, aged 49 +/- 10 years) with chronic hepatitis C alpha-interferon (IFN-alpha) therapy (Intron A, Schering Plough) at a dose of 3 x 10(6) units subcutaneously 3 times a week. The treatment was scheduled for 24 consecutive weeks. Each patient had had stable renal function for at least 12 months prior to IFN-alpha therapy (mean serum creatinine, SCr, 121 +/- 38 mmol/l). Fourteen patients were receiving cyclosporin-A (CsA)-based immunosuppression and 2 patients were on conventional therapy. The patients' SCr was checked every 2 weeks while on IFN-alpha, or weekly if it increased more than 15% from baseline. IFN-alpha was withdrawn if SCr increased more than 25% from baseline, in which case a kidney biopsy was performed. Six patients experienced either acute (n = 5) or subacute (n = 1) renal failure within 7-24 weeks after the onset of IFN-alpha therapy. Their mean SCr increased from 105 +/- 31 to 207 +/- 63 mmol/l (p = 0.02) with de novo proteinuria in 1 case (1 g/day) and an increase in preexisting proteinuria in 2. The other 3 patients did not develop proteinuria. In each case, histological study showed diffuse interstitial edema associated with dilation of the peritubular capillaries, whereas mild inflammatory infiltrates were present in only 3 cases and mild glomerular lesions were not always found (glomerular ischemia, mesangial hypertrophy). There were no vascular lesions. IFN-alpha was withdrawn in these 6 patients, in association with methylprednisolone pulses in 5 cases. Renal function improved in 2 cases, stabilized in 1 and progressed to end-stage renal failure in 3 within 4-12 months. Four of these patients had iterative renal biopsies which showed diffuse interstitial fibrosis in each case. The patients who developed renal failure did not statistically differ at the start of the study from those who did not, with respect to the following: baseline immunosuppression, HLA matching, total peripheral blood lymphocyte count or peripheral blood lymphocyte subtypes. IFN-alpha therapy was associated with acute or subacute renal failure in 37% of the patients. The most prominent histological finding was diffuse interstitial edema of rapid onset, without signs of cellular or vascular rejection. In conclusion, we do not recommend IFN-alpha therapy for KT patients with chronic hepatitis C, until the mechanisms of the subsequent renal failure are better understood.