One hundred and eight children presenting with Mycoplasma pneumoniae infection were assessed during the acute illness and followed for three years. The incidence of wheezing with the acute infection (40%) was greater than expected in a normal childhood population. The initial illness precipitated wheezing for the first time in some subjects but others wheezed only with the acute illness. In non-asthmatic subjects significant bronchodilator responsiveness was present one month after infection. Children given erythromycin during the first seven days of their illness had a significantly shorter fever duration compared with those treated inappropriately. No significant effects of treatment were noted on pulmonary function three years later but non-asthmatic children had abnormal mean forced expiratory volume in one second and forced expiratory flow after 50% of the expired vital capacity compared with 64 healthy controls. These findings indicate impaired function three years after initial infection.