Twenty-one cases (five new and 16 literature) of varicella pneumonia of pregnancy were retrospectively reviewed to evaluate the benefits and risks of intravenous acyclovir on maternal and fetal outcomes. All women were in their second (12 cases) or third (nine cases) trimester. Mean gestational ages at the onset of pneumonia and time of delivery were 27 and 36 weeks, respectively. Twelve patients required mechanical ventilation. The mean duration of treatment was 7 days. No definite adverse drug effects were noted. Three women (14%) died of uncontrolled infection or complications. Two infants died (whose mothers also died): One was stillborn at 34 weeks' gestation, and the other died from prematurity shortly after birth at 26 weeks. No child was born with features of congenital varicella syndrome, and none developed active perinatal varicella infection. Onset of pneumonia during the third trimester was a risk factor associated with fatal maternal outcome. Intravenous acyclovir may reduce maternal morbidity and mortality associated with varicella pneumonia occurring during pregnancy, and appears to be safe for the developing fetus when given during the latter trimesters.