The results are reported here of a long-term double-blind controlled clinical trial of disodium cromoglycate (D.S.C.G.) and isoprenaline, D.S.C.G. alone, isoprenaline alone, and a placebo given as a powder for inhalation in the treatment of severe bronchial asthma. At the end of one year 16 out of 20 patients on D.S.C.G.-isoprenaline remained on the allocated capsules, compared with 10 out of 15 on D.S.C.G., 5 out of 20 on isoprenaline, and 3 out of 19 taking the placebo. The differences between each of the D.S.C.G.-isoprenaline and D.S.C.G. regimens compared with the isoprenaline and placebo regimens were statistically significant. After eight weeks on four capsules a day the patients in each group were allocated at random so that half continued on full dosage and half on a reducing regimen. At the end of the year there was no significant difference in the failure rate between patients allocated the full dosage and the patients on the reducing dosage. The capsules were well tolerated and toxicity to D.S.C.G. was not observed.