OBJECTIVE: To evaluate the efficacy of low-level laser therapy (LLLT) applied to acupuncture points on the knee joint in combination with exercise and advice in patients with knee osteoarthritis. DESIGN: Randomised, double-blind, comparative clinical trial. PARTICIPANTS: Forty-nine patients with knee osteoarthritis were assigned at random into two groups: active laser group (n=26) and placebo laser group (n=23). INTERVENTION: Using a gallium aluminium arsenide laser device, patients received either active or placebo LLLT at five acupuncture points on the affected knee during nine sessions. OUTCOME MEASURES: Patients were assessed using a visual analogue scale (VAS) and the Saudi Knee Function Scale (SKFS) at baseline, the fifth treatment session, the last treatment session, 6 weeks post intervention and 6 months post intervention. RESULTS: VAS scores showed a significant improvement in the active laser group compared with the placebo laser group at 6 weeks post intervention [mean difference -1.3, 95% confidence interval (CI) of the difference -2.4 to -0.3; P=0.014] and 6 months post intervention (mean difference -1.8, 95% CI of the difference -3.0 to -0.7; P=0.003) using the independent samples test. SKFS scores also showed a significant improvement in the active laser group compared with the placebo laser group at the last treatment session (median difference -15, 95% CI of the difference -27 to -2; P=0.035) and 6 months post intervention (median difference -21, 95% CI of the difference -34 to -7; P=0.006) using the Mann-Whitney U test. CONCLUSIONS: The results demonstrate that short-term application of LLLT to specific acupuncture points in association with exercise and advice is effective in reducing pain and improving quality of life in patients with knee osteoarthritis.