Background/Purpose Narrowband UVB phototherapy is a common treatment modality in psoriasis and atopic dermatitis, but evidence of its actual effect in clinical setting is sparse. Our aim was to assess the effectiveness and costs of narrowband UVB phototherapy in psoriasis and atopic dermatitis in clinical setting. Methods We observed 207 psoriasis patients and 144 atopic dermatitis patients in eight centers. SAPASI, PO‐SCORAD, and VAS measures were used at baseline, at the end, and 3 months after the narrowband UVB phototherapy course. Quality of life was measured using Dermatology Life Quality Index (DLQI), and costs were assessed using a questionnaire. Results In both psoriasis and atopic dermatitis, the DLQI and Self‐Administrated PASI (SAPASI)/Patient‐Oriented SCORAD (PO‐SCORAD) improved significantly and the results remained improved for at least 3 months in both groups. Alleviation of pruritus correlated with better quality of life in both patient groups. We reported slight redness and burning side effects which were due to lack of MED testing. Self‐administered tools proved to be useful in evaluating pruritus and severity of the disease in psoriasis and atopic dermatitis. Mean patient costs were 310 € and 21 hours of time, and mean costs for the healthcare provider were 810 €. Conclusion In psoriasis, narrowband UVB is a very efficient treatment in clinical setting, whereas in atopic dermatitis, more studies are needed to determine the best dosage.