This open labelled, group comparative, cross over study, was carried out to analyze the efficacy of two dosages of Nedocromil sodium (NS) in the prophylaxis of Exercise Induced Asthma (EIA). 13 athletes (6 males, 7 females), aged: 12-38 years, were studied measuring their EIA by forced spirometries prior and at 1, 3, 5, 10, 15 and 30 minutes post treadmill test. Four mg of NS were administered via inhalation chamber 30 minutes prior to the second test; prior to the third one;: NS at a dose of 4 mg every 8 hours during four weeks plus 4 mg of NS 30 minutes before it. In the baseline exercise test, a 33.6 +/- 10.5 percent decrease of FEV1 was observed; 12.6 +/- 11.4 in the second one, and 9.7 +/- 9.76 in the third test. A protection index higher than 30% was obtained in 9 cases of the second test (81.5 +/- 16.2%), and in 10 of the third one (77.2 +/- 15.3%). Nedocromil sodium (4 mg prior to exercise) is effective for EIA prophylaxis. Although no significant differences exist between both dosages, a greater protection index is reached after NS sustained administration.