To study the effect of age on ultimate outcome of febrile illness, the authors prospectively studied 1,202 adult patients who came to an emergency room/walk-in clinic setting with temperatures of 101.0 degrees F (38.3 degrees C) or more. The patients were divided into four age categories: 17 to 40 years, 40 to 59 years, 60 to 79 years, and 80 years old or older. Advancing age was significantly (P less than 0.0005) associated with more serious disease, a higher rate of bacterial pathogen isolation, and a higher rate of life-threatening or deadly consequences. Of patients 17-39 years old, 58.2 per cent had viral syndromes, otitis media, or pharyngitis as the causes of fever. Of patients aged 40-59 years, only 20.7 per cent had one of these diseases. However, of patients 60 years old or older, only 4.1 per cent (15 of 370) had viral syndromes, otitis media, or pharyngitis, and the overall rate of hospitalization for this group was 92.5 per cent. The authors conclude that febrile patients 60 years old or older seen in emergency room/ambulatory care settings are extremely likely to have serious diseases. Caution should be exercised before concluding that their fevers are of benign origin.