UVB phototherapy of atopic dermatitis was investigated in two paired-comparison studies. In study I, 17 patients were treated for 8 weeks with ultraviolet B radiation (UVB) on one half of the body and with visible light (placebo) on the other. A severity score based on eight clinical variables was determined before, during and after treatment. The total score, pruritus score and overall evaluation score were significantly better on the UVB treated sides than on the placebo treated sides (P less than 0.001). In 13 patients the UVB treated side healed or improved considerably, while the placebo treated side improved considerably in one patient only. In study II, 25 patients were treated for 8 weeks with 0.8 of the minimal erythema dose (MED) of UVB on one half of the body and with 0.4 MED UVB on the other. No statistically significant differences between these dosage regimens were found.