Thirteen patients with herpesvirus infections who were unresponsive to at least 72 h of intermittent acyclovir administration received high-dose continuous infusion. Steady-state concentrations were maintained at between 20 and 98 mumol/liter. Of 12 patients who had continuous infusion for greater than 5 days, 7 (58%) resolved their infections, as determined by clinical and virologic parameters, suggesting that continuous infusion may succeed in some patients who do not respond to conventional therapy.