The authors adhered to many of the methodologic criteria recommended for generating clinical decision rules (11–12). The strengths of their approach include provision of a clear definition of the outcome and the predictive variables, blind assessment of the predictive variables, description of the mathematical techniques used to derive the rule, description of details of the study site and patient sample, description of the results of the rule, and provision of suggested clinical courses of action that are based on the rule. However, as the authors acknowledge, they have not validated the rule in a group of patients different from the derivation cohort. External validation may be particularly important for this rule because it was derived by using combined data from 2 epidemiologically distinct groups of patients with SARS in Hong Kong. The group from the United Christian Hospital, which made up two thirds of the derivation cohort and primarily included patients from the unique Amoy Gardens housing complex outbreak, had a much lower proportion of patients who were health care workers (8.8% vs. 59.2%) and patients who provided a history of contact with another patient with SARS (11.4% vs. 87.5%) than did the group from the Prince of Wales Hospital. These characteristics of the United Christian Hospital cohort also seem to differ from those of patients in other major SARS-affected areas of the world in 2003. For example, in Toronto and Singapore, SARS was primarily a nosocomial illness, largely restricted to health care workers, patients, and visitors exposed in affected hospitals and, to a lesser extent, to other persons who had close contact with known or suspected patients with SARS in household settings (13–14). The outbreak involving Amoy Gardens has been widely discussed because of its peculiar epidemiologic characteristics (15–16). The generalizability of epidemiologic variables that emerged as important components of a clinical decision rule derived from this population remains to be determined.