Although influenza virus continues to cause annual epidemics of respiratory diseases, surprisingly little is known about the frequency and clinical course of influenza among adult patients with cancer. During the 1991-92 influenza epidemic in Houston, Texas, we followed all adult BMT recipients hospitalized at M.D. Anderson Cancer Center. None of these 68 patients had received prophylaxis for influenza. Influenza virus type A was isolated from 8 (29%) of 28 BMT recipients with an acute respiratory illness. Five of these infections were acquired in the hospital. All 8 patients presented with an upper respiratory tract illness. In 6 patients, the infection was complicated by pneumonia. The frequency of influenza was similar among autologous (5 of 18) and allogeneic (3 of 10) BMT recipients. The risk of developing pneumonia was not related to the type of transplant or to the engraftment status. All patients received broad-spectrum antibiotics. The 2 patients who did not develop pneumonia also received amantadine. The mortality with pneumonia was 17%. During community outbreaks, influenza is a frequent cause of acute respiratory illness among hospitalized adult BMT recipients and is frequently complicated by pneumonia. Studies are needed to define the optimal means of preventing and treating influenza in BMT recipients.