Background: Vehicle-controlled studies have demonstrated the efficacy and safety of tacrolimus ointment for patients with atopic dermatitis. Objective: This study was undertaken to compare 0.03% and 0.1% tacrolimus ointment with 0.1% hydrocortisone-17-butyrate ointment, a midpotent to potent topical corticosteroid, in the treatment of adult patients with moderate-to-severe atopic dermatitis. Methods: Patients applied ointment twice daily to all affected areas for 3 weeks in this multicenter, randomized, double-blind, parallel-group study. The primary endpoint was the modified eczema area and severity index (mEASI) mean area under the curve as a percentage of baseline. Results: Five hundred seventy patients were randomized and received treatment. Discontinuations included 22 of 193 patients from the 0.03% tacrolimus group, 22 of 191 patients from the 0.1% tacrolimus group, and 17 of 186 patients from the hydrocortisone butyrate group. The median mEASI mean area under the curve as a percentage of baseline was 47.0%, 36.5%, and 36.1% for patients who received 0.03% tacrolimus, 0.1% tacrolimus, and 0.1% hydrocortisone butyrate, respectively. There was no statistically significant difference between 0.1% tacrolimus and 0.1% hydrocortisone butyrate; however, the lower improvement in mEASI for 0.03% tacrolimus was statistically significant when compared with 0.1% tacrolimus (P < .001) or hydrocortisone butyrate (P = .002). Skin burning and pruritus at the application site showed a higher incidence in the tacrolimus treatment groups than in the hydrocortisone butyrate group (P < .05). Laboratory parameters showed no treatment differences and no marked changes over time. Conclusions: The efficacy of 0.1% tacrolimus ointment was similar to that of 0.1% hydrocortisone butyrate ointment and was lower for 0.03% tacrolimus ointment. No serious safety concerns were identified. (J Allergy Clin Immunol 2002;109:547-55.)