Triamcinolone acetonide, a water-insoluble corticosteroid preparation in a Freon-propelled metered-dose aerosol, was administered via a specially designed nebulizer to 22 adult patients with severe, chronic asthma. All patients had required oral corticosteroid therapy, daily, for several years, in order to control their incapacitating obstructive airways disease. During 4 wk of treatment with triamcinolone acetonide, taken in 4 daily doses totaling 8 to 28 inhalations (400-1,400 mcg) per day: (1) asthma remained under satisfactory control; (2) oral corticosteroids were stopped in 19 patients and reduced in 2, and 1 patient withdrew from the study; (3) adrenal cortical function returned to normal or near normal levels; (4) spirometric and plethysmographic measurements improved significantly; (5) daily self-measurements of peak expiratory flow showed a marked improvement in median weekly levels, as well as a reduction in daily variability. No immediate undesirable side effects were noted. These observations indicated that more extensive applications of topical corticosteroid therapy are justified as an effective, convenient, and relatively safe method for the preventive management of severe, persistent asthma.