The authors describe a prospective study of 420 patients aged 3 months to 5 years who presented to a primary pediatric clinic owing to fever > or = 38 degrees C and signs of pharyngitis and were not treated with antibiotics in the preceding week. Throat cultures and blood antistreptolysin O (ASO) titers were examined. In group A beta-hemolytic streptococcus (GABHS)-positive patients, a second ASO sample was obtained 2-3 weeks later. Positive throat cultures to GABHS were found in 61 of 415 patients (14.7%) (five patients were lost to follow-up). Thirty-three of these (54.1% of the culture-positive group and 8% of the total study group) had the streptococcal infection with elevated ASO titers. The incidence of both true infection and carrier state gradually increased with age. Nevertheless, true streptococcal pharyngitis was found even in patients younger than 1 year and its percentage related to carriers did not increase with age and was > or = 50% in all age groups up to 4 years. The authors conclude that true GABHS pharyngitis may present in the first year of life.