In the summer of 1989, ozone levels in the ambient air were regularly found to be increased in The Netherlands. The question was studied whether this affected pulmonary function of children aged 7 to 12 years practising sports in the open air. Before and after sessions of training in the open air, the peak expiratory flow rate was determined using peak flow meters (mini-Wright). This was done on 37 different days with varying ozone concentrations. Measurements were carried out on four or more days in a total of 65 children. No correlation between ozone concentration during training and differences between pre- and post-training peak flows was found. No correlation was found either between the maximal ozone concentration on a given day and the peak flow at the end of training on that day. However, a negative correlation was found between the maximal ozone concentration on the day preceding the pulmonary function tests and the peak flow after the training.