Nedocromil sodium (Ned) 4 mg, sodium cromoglycate (SCG) 10 mg, and placebo were compared for their efficacy in preventing exercise-induced asthma. Nineteen asthmatic children aged six to 15 years performed a treadmill exercise test before and 20' after a single dose of drug in a double-blind trial. Both active drugs performed significantly better than placebo; in fact the exercise challenge resulted in a mean maximum fall in FEV1 of 26.1 +/- 14.9% after placebo, but only of 14.6 +/- 11.5% after SCG (P < 0.05), and 11.0 +/- 12.4% after Ned (p < 0.01). Measurements of PEFR gave similar results, while the effect of treatment on FEF 25-75 was significant for Ned alone (p < 0.05). Direct comparison between Ned and SCG at different time points demonstrated significant differences in FEV1 at 1 min (p < 0.05) with a better overall performance of Ned. In individual patients, complete protection was provided in 9 patients with SCG, in 14 patients with Ned and in 2 with placebo. No side effects were observed. This study suggests that at the dosages used there are only slight differences between SCG and Ned activity in the prevention of exercise-induced asthma.