Systematic monitoring of infants and children in a day-care center revealed infection with Mycoplasma pneumoniae to be more common than expected. Most of these infections were asymptomatic (74%) or associated with mild, nonspecific coryza and cough. Infected children ranged in age from 2 months to 8 years. Complement-fixing and growth-inhibiting antibodies in serum tended to wane rapidly and reinfection was detected in five children after one and one-half to 3 years. In vitro lymphocyte studies revealed antigen-reactive cells were detectable in few of the children under age 4, but thereafter lymphocyte reactivity was found in 53%. These findings suggest that recurrent, unsuspected M. pneumoniae infections occur during infancy and early childhood and that pneumonic disease, common above age 10 years, is an expression of increasing host immune response to the organism.