To identify valid measures for preventing outbreaks of severe acute respiratory syndrome (SARS) among protected healthcare workers in isolation units. Architectural factors, admitted SARS cases and infection of healthcare workers in different isolation wards between January 30 and March 30, 2003 were analyzed. Four types of isolation wards were analyzed, including the ward where the thirty-first bed was located on the twelfth floor, the laminar flow ward in the Intensive Care Unit where the tenth bed was located on the fifteenth floor, the ward where the twenty-seventh bed was located on the thirteenth floor of the Lingnan Building, and thirty wards on the fourteenth to eighteenth floors of the Zhongshan Building. The ratios (m(2)/m(3)) of the area of the ventilation windows to the volume of the rooms were 0, 0, 1:95 and 1:40, respectively. Numbers of SARS cases in the wards mentioned above were 1, 1, 1 and 96, respectively. Total times of hospitalization were 43, 168, 110 and 1272 hours, respectively. The infection rates of the healthcare workers in the areas mentioned above were 73.2%, 32.1%, 27.5% and 1.7%, respectively. The difference in the infection rates was of statistical significance. Isolating SARS cases in wards with good ventilation could reduce the viral load of the ward and might be the key to preventing outbreaks of SARS among healthcare workers along with strict personal protection measures in isolation units.