Varicella pneumonia in adults is considered to be a serious complication, with mortality rates of 9-50%, but the true incidence and clinical course is not known. We therefore studied all adult patients in Stockholm County hospitalized during the period 1980-1989 with varicella. 36/305 (12%) varicella patients admitted to hospital were diagnosed as having pneumonia, corresponding to a mean incidence of 0.32/100,000 inhabitants per year. In most patients the pneumonia had a mild to moderate clinical course and no deaths occurred (mortality rate 0%, CI95 [0, 9.7]). However, 13 patients had a severe tachypnoea (> or = 30 breaths/min) and 4 of these required intensive-care treatment. Ten patients were treated with acyclovir, in most cases combined with corticosteroids. We conclude that the incidence of severe varicella pneumonia is low in the adult population, and that the mortality rate of this complication is probably lower than previously described.