Purpose: Cutaneous features of dermatomyositis (DM) strongly suggest that ultraviolet (UV) radiation plays an important role in the pathogenesis of the disease. However, the incidence and the nature of photosensitivity in this disorder have not been established. The aim of this study was to investigate the UVB (290–320 nm) minimal erythema dose (MED) in DM patients in comparison with those in lupus erythematosus (LE) and healthy controls. Methods: Non-irradiated back skin of 75 Caucasians with skin types II and III according to the Fitzpatrick classification were present in three different subject groups and tested for photomanifestation on non-irradiated suprascapular back skin with an ETG-1 Erythemtester. The first group included 19 DM patients, the second 30 patients with LE, and the third 26 healthy control volunteers. The MEDs were determined 24 h after irradiation adjusted according to skin type. Results: Nine of the 19 DM patients (47.4%) demonstrated reduced MEDs to UVB radiation. Seven DM patients (36.8%) had a history of increased cutaneous photosensivity and four of these (21.1%) reported diseased aggravation after sun exposure. Both the DM and LE patient groups showed reduced MED to UVB radiation (P<0.05) compared with the control group (19.2%). Increased erythemal sensitivity to UVB irradiation was found more frequently in patients with systemic LE and cutaneous discoid LE, than in those with subacute cutaneous LE. Conclusion: DM patients, similar to those with LE, showed a significantly reduced MED to UVB irradiation compared with healthy persons.