A rapid latex agglutination (LA) method was evaluated in 2401 consecutive pediatric patients presenting to an emergency service with suspected group A beta-hemolytic streptococcal pharyngitis. LA tests were performed by the treating physicians, who were not blinded to the clinical condition of the children and who made therapeutic decisions based on the results of the tests. When compared with anaerobic culture, the LA method had a sensitivity of 91%, a specificity of 82%, and a positive predictive value of 43%. There was a marked seasonal variation in the positive predictive value: 62% in winter and 16% in summer. However, even in peak streptococcal pharyngitis season (January to March), basing therapy on a positive LA test leads to the unnecessary treatment of a large number of patients. Therefore, we cannot recommend the routine performance of this test by all practitioners in all clinical settings.