Two children who developed acute renal failure in the immediate post-renal transplantation period are presented. Each was immunosuppressed with cyclosporin and was also receiving oral acyclovir for prophylaxis against cytomegalovirus infection. Renal biopsy findings suggested drug toxicity. Discontinuation of acyclovir coincided with reversal of renal impairment. As cyclosporin levels were at the lower end of the therapeutic range, we believe acyclovir to be the likely causative agent.