Twenty-five immunocompromised children with varicella were treated with oral acyclovir 800 mg, five times daily for 7 days. Two patients were transferred from the oral to the intravenous route: one had signs of varicella pneumonitis on routine X-ray, the other had continuing new lesion formation on day 4 of oral treatment. The disease healed in all patients, with no other evidence of dissemination. In an historical placebo treated group, 12 of 25 patients were transferred to intravenous acyclovir. The reduction to two of 25 is statistically significant (P < 0.01). The mean peak plasma acyclovir concentration in these patients was 6.56 mumol/l. Mild, self-limiting diarrhoea in nine patients was the only adverse event considered to be related to acyclovir. It is concluded that immunocompromised children with varicella can be treated safely and effectively with oral acyclovir. All patients should be observed closely by a physician.