Fifty children with a previous history of Mycoplasma pneumoniae respiratory tract infection were assessed clinically, and pulmonary function tests carried out after an interval ranging from 1 1/2 to 9 1/2 years (median 2 1/2). 23 suffered from recurrent wheezy bronchitis or asthma, and in 5 the index illness appeared to precipitate the wheezing tendency. All were symptom-free when respiratory function tests were performed. Simple tests of ventilatory function (PEFR, FEV, and FVC) were within normal limits. Increased bronchial reactivity after exercise (a fall in PEFR greater than 15% resting value) was demonstrated only in children known to have asthma. Maximum expiratory flow rates in air at 50% of vital capacity (V mas50) were within the normal range in all patients with the exception of two. The response in flow rate at 50% of vital capacity after inalation of an 80% helium and 20% oxygen mixture delta V max50) was reduced (P less than 0.001) in asymptomatic patients with a history of M. pneumoniae respiratory infection, when compared with normal data from 48 healthy schoolchildren without a background of significant respiratory illnesses. These findings indicate impairment of small airways function, even in totally symptom-free children in the study group.