Background: UVA‐1 has been shown to be effective in the treatment of patients with atopic dermatitis. However, its optimal therapeutic conditions are not yet fully established.

Methods: In an open prospective study we retrospectively compared the effect of 4 weeks therapy (32 patients) with the effect of the usual 3 weeks therapy (29 patients) in patients with atopic dermatitis, using a medium dose UVA‐1 cold light (45 J/cm²), 5 days a week.

Results: Scoring atopic dermatitis index (SCORAD) and dermatology life quality index (DLQI) quality of life indexes improved significantly during both 3 and 4 weeks UVA‐1. Patients who were treated for 4 weeks showed a superior improvement of the SCORAD index [23.12 points, 95% confidence interval (CI) 16.09–30.16, vs. 13.32 points, 95% CI 5.61–21.04, P=0.059], and the DLQI (5.41 points, 95% CI 2.38–7.88, vs. 3.86 points, 95% CI 1.88–5.84, P=0.360), compared with patients who were treated for 3 weeks. However, the differences did not reach statistical significance. Only patients who were treated for 4 weeks were able to maintain their improvement 6 weeks after therapy. In both groups 50% of patients had intermittently used mild topical corticosteroids in the follow‐up period.

Conclusion: Extension of UVA‐1 therapy from 3 to 4 weeks results in a clinically relevant improvement of the outcome, and more prolonged therapeutic effects, measured by the SCORAD index.