Abstract: Objective To investigate the epidemiological features of nosocomial infection of severe acute respiratory syndrome (SARS) in the medical staff of a hospital. Methods The medical staff associated with nosocomial infection of SARS in a hospital were investigated with standard procedures of epidemiological survey. Results The source of SARS infection was identified as the husband of a tumor patient who received chemotherapy in the Department of Oncology, during which they maintained intimate contact for at least two weeks when the husband had typical symptoms of SARS. The tumor patient had no any signs of SARS before her death. The mean latent period of SARS was 12.6 days in the 8 nurses transferred temporarily from other departments to assist in the management of the tumor patient. All 57 persons with close contact with the 12 medical staff suffering SARS in the latent period or with one having early-stage SARS did not show any signs of SARS during one month for observation in isolation. Conclusions SARS patients may have no or very limited infectivity during the latent period. Infection of SARS might be concerned with the intensity and persistence of contact with the infectious sources. The pathogenesis of SARS might involve the immune status of the potential patients, and immunodeficient individuals are at the risk of becoming asymptomatic carriers of SARS virus and the infectious sources.