BACKGROUND Blue light has no known toxic effects on human skin, but reduces the proliferative capacity of keratinocytes in vitro. We therefore investigated the efficacy of blue light in the treatment of psoriasis vulgaris (PV). METHODS Forty patients with mild to moderate PV and bilateral plaques were assigned to two groups. Group 1 (n = 20) received irradiation at home with blue light (light-emitting diode, LED, emission maximum: 420 nm) once daily for 4 weeks. In parallel, group 2 (n = 20) performed irradiations with another blue light device (LED emission maximum: 453 nm). The contralateral control plaques remained untreated in both groups. RESULTS Thirty-seven patients completed the trial. The main study parameter, the difference of Local Psoriasis Severity Index (LPSI) scores of the irradiated plaques compared to the control plaques, showed statistically significant improvement after 4 weeks of treatment in both groups [group 1 (420 nm): n = 17, p = 0.04; group 2 (453 nm): n = 20, p = 0.04]. Accordingly, plaque status as assessed by both the physicians and the patients improved continuously during the 4 weeks of treatment and steadily declined thereafter. CONCLUSION Blue light appears to be a promising treatment modality in PV that warrants further evaluation in larger studies.