Narrow-band TL-01 ultraviolet B phototherapy (TL-01) is an effective and widely used treatment for many skin diseases. The purpose of the investigation was to assess the risk of skin cancers in patients treated with TL-01 phototherapy who have not received any other phototherapy modalities. This cohort study included 4,815 TL-01 treated patients in Finland with psoriasis or atopic dermatitis. Clinical information was collected from the hospital records and linked with Finnish Cancer Registry data. The follow-up started from the first TL-01 treatment and the mean follow-up time was 8.4 years. Standardized incidence ratios were calculated for basal cell carcinoma, cutaneous melanoma, and squamous cell carcinoma. The standardized incidence ratio for basal cell carcinoma was 2.5 (95% confidence interval 1.8-3.5), for cutaneous melanoma 4.0 (95% confidence interval 2.1-6.8) and for squamous cell carcinoma 3.7 (95% confidence interval 1.7-7.0). For basal cell carcinoma and squamous cell carcinoma, the standardized incidence ratios remained similar during the whole follow-up time while the standardized incidence ratio for cutaneous melanoma was markedly higher during the first 5 years of follow-up. In conclusion, an increased incidence of skin cancers was observed among TL-01 treated patients. It should be confirmed in the future whether the skin cancer risk of TL-01 phototherapy will remain high in a longer follow-up.