Of 19 children treated with high-dose intravenous acyclovir, 7, all of whom had encephalitis, also had a restricted fluid intake. Of these, 3 experienced nonoliguric renal insufficiency (plasma creatinine up to 176, 250 and 351 mol/l, respectively) which resolved within 1 week of discontinuing acyclovir. Renal function was stable in the remaining 4 patients in this group and in the 12 children treated with acyclovir but without fluid restriction. We suggest the decreased renal function resulted from intratubular acyclovir crystalluria.