A device for phototesting patients prior to narrowband phototherapy is described. One hundred and fifty patients (130 with psoriasis and 20 with eczema) of skin types I-IV were phototested on the forearm and 22 on both forearm and back. The minimal erythema dose (MED) was judged visually 24 h after irradiation, and in those patients who were tested at two body sites, objective measurement of the erythema was made using a reflectance instrument. The MED values on the arm showed a fivefold range. There was no significant association between skin type and MED. The MED values on the arm were significantly higher than those measured on the back, although the differences were small in the majority of cases. No significant difference was found between the slopes of the dose-response curves measured on the arm and on the back.