In view of the high incidence of exercise-induced asthma (EIA) in asthmatic adolescents and adults, the aim of the present study was to examine the preventive effect of nedocromil sodium administered prior to an exercise provocation test. The study involved 30 adolescents with EIA, aged between 14 and 19 years, who attended gymnastics classes at the Children's Hospital and Aerobics School. Baseline spirometry, six-minute treadmill running and spirometry for 7-15 minutes post-exercise were performed. During the same week and under identical temperature and humidity conditions, the same test was carried out with 4 mg nedocromil sodium (two inhalations) administered 30 minutes before exercise. Original post-exercise FEV1 values were compared with the findings after administration of nedocromil sodium. Seventeen subjects (56%) were totally protected (< 5% decrease in FEV1) and 13 subjects (44%) were partially protected (< 10% decrease in FEV1). EIA persisted in the fifteen subjects who received placebo pretreatment. Nedocromil sodium, administered 30 minutes prior to exercise, was effective, providing an opportunity to enhance the quality of life of the asthmatic patient troubled by exercise-induced symptoms.