In a retrospective survey of sera collected from 126 patients under the age of 10 years, seroreactivity was first detected at 1 year of age when the geometric mean titer rose from 12 to 24. This activity increased to a titer of 102 in the 4- to 6-year-old group and was maintained in the 7- to 9-year-old group. At the time of serum collection, at least 25% of those sampled had a titer of 256 or greater, a level currently thought to be presumptive evidence of infection at some undetermined time. No difference in the geometric mean titer could be ascertained when the population was divided by clinical diagnosis or by sex. No seasonal variation was observed. Of 35 paired sera from children under the age of 5 years, three fourfold rises were observed. One rise to a titer of 128 was detected in an 11-month-old girl with a clinical diagnosis of bronchiolitis. The second rise in a 5-year-old boy with pneumonia was an increase from 256 to 2,048. The third rise in a 6-month-old with pneumonia did not meet the currently accepted level necessary for confirmed diagnosis. These data suggest that infection with Legionella pneumophila, or a closely related agent, is common in this population, occurring before 9 years of age, and may be a cause of mild respiratory disease in infants and children.