Sixteen children completed a randomized placebo-controlled double-blind study comparing the efficacy of pre-treatment with (1) 20 mg sodium cromoglycate (SCG) powder delivered by Spinhaler, (2) 2 mg SCG delivered by pressurized aerosol and (3) placebo. The drug was administered 15 min before the 6 min of treadmill-running performed at the same time of day on 3 consecutive days in a climate chamber with a temperature of 23 degrees C and a relative humidity of 40%. All the patients, none of whom were taking systemic steroids, had shown a post-exercise fall in peak expiratory flow (PEF) of more than 15% from baseline when tested one day prior to the study. On the day with no pre-treatment the average post-exercise fall in PEF was 46% (+/- 4) (mean +/- s.e. mean). After pre-treatment with SCG powder the average fall in PEF was 21% (+/- 3), after pre-treatment with SCG aerosol the fall was 22% (+/- 3) and after placebo a post-exercise fall of 31% (+/- 5) was seen. Both forms of SCG were significantly better than pre-treatment with placebo. Comparing no pre-treatment with pre-treatment with placebo revealed a statistically significant difference (P less than 0 . 001) indicating a placebo effect. It is concluded that this study design has a marked placebo effect. Both pre-treatment with 2 mg SCG delivered by pressurized aerosol and 20 mg delivered by Spinhaler is superior to placebo in conferring protection against exercise-induced asthma (EIA). No bronchodilating effect was seen in the first 15 min after administration of the drug.