Between 1 and 22 March 2003, a nosocomial outbreak of Severe Acute Respiratory Syndrome (SARS) occurred at the Communicable Disease Centre in Tan Tock Seng Hospital, Singapore, the national treatment and isolation facility for patients with SARS. A case-control study with 36 cases and 50 controls was conducted of factors associated with the transmission of SARS within the hospital. In univariate analysis, contact with respiratory secretions elevated the odds ratio to 6·9 (95% CI 1·4–34·6, P=0·02). Protection was conferred by hand washing (OR 0·06, 95% CI 0·007–0·5, P=0·03) and wearing of N95 masks (OR 0·1, 95% CI 0·03–0·4, P=0·001). Use of gloves and gowns had no effect. Multivariate analysis confirmed the strong role of contact with respiratory secretions (adjusted OR 21·8, 95% CI 1·7–274·8, P=0·017). Both hand washing (adjusted OR 0·07, 95% CI 0·008–0·66, P=0·02) and wearing of N95 masks (adjusted OR 0·1, 95% CI 0·02–0·86, P=0·04) remained strongly protective but gowns and gloves had no effect.