The usefulness of clinical, radiologic, and physiologic characteristics to identify pathogens was assessed in 90 infants aged two through 12 weeks, presenting to an outpatient clinic with an acute respiratory infection. Eighty-four cases had cultures or rapid diagnostic tests for RSV, of which 25 were positive. Eighty-two infants had cultures or rapid diagnostic tests for Chlamydia, of which 16 were positive. Additional respiratory pathogens identified included parainfluenza (6 cases), rhinovirus (3 cases), pertussis (2 cases), and CMV (1 case). Multiple pathogens were identified in four cases: Chlamydia and RSV (2 cases), Chlamydia and parainfluenza (1 case), and Chlamydia and CMV (1 case). Clinical characteristics other than the need for hospitalization were not useful predictors of specific pathogens. X-rays were obtained in 20 (80%) of the RSV infections, 13 (81%) of the Chlamydia infections, six of the parainfluenza infections, two pertussis infections, and 25 cases without identification of the pathogen. X-ray findings could not distinguish between patients with or without a pathogen or between the pathogens. Severe findings were present in 28% (11/40) of cases with a pathogen identified, compared to 12% (3/26) of cases without a pathogen identified (NS). Moderate findings were present in 58% (23/40) of cases with a pathogen identified compared to 62% (16/26) of cases without a pathogen identified. Slight/negative findings were present in 15% (6/40) of cases with a pathogen identified, compared to 27% (7/26) of cases without a pathogen identified. Pulse oximetry was done in 30 cases, 22 of which had a pathogen identified (RSV 14, Chlamydia 7, pertussis 1).(ABSTRACT TRUNCATED AT 250 WORDS)