The aim of this double-blind study was to evaluate the effect of low power Ga-Al-As laser treatment on chronic pain related to osteoarthritis of the knee with periarticular tender points. Twenty-nine out-patients with uni- or bilateral osteoarthritis of the knee were included and randomly assigned to treatment with either laser or placebo laser. Fourteen patients received active laser treatment and all patients included completed the study. The effect variables were daily levels of pain, analgesic requirements, palpation tenderness and isokinetic quadriceps strength. Each patient participated in the study for 9 weeks and registered daily level of pain and consumption of analgesics. In weeks 4, 5 and 6 the patients received a total of nine treatments, each of 15 min and administered to periarticular tender points. The dose per treatment was 22.5 joule. No significant differences in any of the effect variables were found between the two groups before, during or after treatment. With regard to the patients' overall assessment there was a clearly demonstrable positive effect of treatment in both groups. This is likely to be due to a placebo effect.