A case-control study of exercise-induced airways narrowing (EIAN) and exposure to environmental tobacco smoke has been carried out in Mataró, Spain. This study followed a prevalence study on EIAN performed on a random sample of 2,056 schoolchildren aged 9-14 years. Cases were 136 children having a decrease in postexercise peak expiratory flow rate equal to or greater than 15%; two controls per case, matched by sex, age, and classroom, were selected among children with a negative EIAN test. A complete history of environmental tobacco smoke exposure was collected from the parents by means of a questionnaire. Exposure from the mother was associated with EIAN in children (odds ratio = 2.23, 95% confidence interval 1.06-4.69) compared with those never exposed. Regarding the duration, intensity, and cumulative exposure, there is an overall tendency for the more heavily exposed to be at greater risk. Other sources of environmental tobacco smoke exposure at home showed no effect on EIAN in children. The results of our study do not seem to be confounded by antecedents of parental asthma or a previous diagnosis of asthma in children, and they suggest that environmental tobacco smoke exposure could play a role as a determinant of the occurrence of EIAN in children.