Mycoplasma pneumoniae infection was monitored in patients with symptoms of acute respiratory tract infection in a village in southeastern Ontario from April 1983 to April 1984. M. pneumoniae was isolated from 51 (48%) of the 106 patients. The incidence began to increase in May 1983, reached a peak in July and declined to normal by mid-August. During the epidemic period M. pneumoniae was detected in 36 of the 43 symptomatic patients. The most prominent features of the outbreak were the considerable intrafamilial attack rate and the high frequency of pneumonia among infected patients. Treatment with tetracyclines and erythromycin reduced the duration of the illness and accelerated the resolution of symptoms.