Chronic hepatitis C is a common cause of viral liver disease in kidney transplant recipients. To assess the efficacy and the safety of therapy with interferon alpha (IFN alpha) in such a population we conducted a prospective study where 16 kidney transplant recipients with chronic hepatitis C received recombinant IFN alpha 3 million units three times weekly scheduled for 24 consecutive weeks. All the patients had stable renal function for at least 1 year (mean serum creatinine 125.4 +/- 41 mumol/l). Fifteen patients had a positive HCV viraemia at the beginning of the study. In 15 patients serum alanine aminotransferase (ALT) levels decreased rapidly and normalized (48 +/- 44 vs 98.5 +/- 46 IU/l; P = 0.0044). ALT remained in the normal range as long as IFN alpha was continued. Serum levels of gamma glutamyl transpeptidase decreased from 129.75 +/- 111.2 to 88 +/- 85 IU/l; P = 0.012). After discontinuation of IFN alpha therapy seven responders relapsed within 1-9 weeks. HCV viraemia assessed 1 month after the end of IFN alpha therapy remained positive in all the patients who scored positive at the beginning, i.e. 15. Side effects of IFN alpha (fatigue, anorexia, weight loss) were frequent leading to four patients dropping out of the study. The haematological tolerance was moderate. The major concern was the increase in serum creatinine (162.5 +/- 57.6 vs 125.4 +/- 41 mumol/l; P < 0.05). In fact only six patients experienced renal failure occurring 45-168 days after the beginning of IFN alpha. Kidney transplant biopsies showed oedema, scarce scattered interstitial inflammatory cellular infiltration and moderate mesangial hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)