Health officials caution that severe acute respiratory syndrome (SARS) may return this fall with the flu season, and the serious disease may be difficult to distinguish from the common malady. Paying attention to public health trends will help you determine how much a diagnosing concern SARS should be in your own ED. More use of rapid diagnostic tests for flu may help, but don't depend too much on them. Confirming a flu diagnosis means you're relatively safe in ruling out SARS. ED physicians may have to be overly cautious this year in identifying cases of potential SARS.