Background: Several studies have demonstrated the efficacy of UVA1 (340-400 nm) phototherapy for patients with severe atopic dermatitis. However, the optimum treatment dose has yet to be determined. Although in seminal investigations high UVA1 doses were used, comparable results were reported in recent studies with a medium-dose regimen. Objective: Our purpose was to compare the efficacy of high-dose with medium-dose UVA1 phototherapy for patients with severe generalized atopic dermatitis. Methods: Ten adult patients with a median baseline SCORAD score of 67 were enrolled in an investigator-blinded, bilateral comparison study. Treatment was given 5 times weekly over a period of 3 weeks on an outpatient basis. Irradiation was performed by exposing one half of the patient's body to high-dose UVA1 (≤130 J/cm²), and the contralateral body side received only half that dose. The clinical response was assessed after 5, 10, and 15 treatments. After completion of the study, patients were followed up for 6 months to evaluate the duration of clinical improvement. Results: All but one patient responded favorably to treatment. High-dose UVA1 led to a decrease of the median SCORAD score by 33.4% after 1 week, 38.4% after 2 weeks, and 34.7% after 3 weeks. The respective values for the medium-dose regimen were 29.7%, 36.4%, and 28.2%. The difference in efficacy between the two dosages remained below the level of significance at all time points. Relapses occurred after a median of 4 weeks. Time of onset and severity of relapse were the same for both doses. Conclusion: Our data support previous uncontrolled observations that medium-dose UVA1 is comparably as effective as high-dose treatment for patients with severe generalized atopic dermatitis. Irrespective of the dose regimen, follow-up examinations revealed early relapse in the majority of patients. (J Am Acad Dermatol 2001;45:503-7.)