The effect of the antiviral drug phosphonoformate (Foscarnet) was studied in 32 patients following renal transplantation. Viral diagnosis was verified in 29 of 33 episodes of suspected clinical virosis. The clinical effect of Foscarnet was good in 12 of 14 primary cytomegalovirus infections. In 5 of 7 varicella-zoster virus, in 4 of 6 secondary cytomegalovirus and in 2 herpes simplex virus (type 1; type 2) infections the effect of the treatment was clinically judged as good. The beneficial effect could in these patients be overestimated as the natural courses of the viral infections were unknown. No clinical side effects of Foscarnet were found. Clinically unimportant changes in s-calcium were noted in 6 patients. The dosage of Foscarnet was increased during the study. At present an initial bolus dose followed by 14 days of parenteral infusions can be recommended.