OBJECTIVE: To examine whether the homeopathic medicine Arnica D30 has an effect on muscle soreness and cell damage after marathon running. METHODS: The subjects were 82 marathon runners from two separate randomised double-blind placebo controlled trials participating in the Oslo Marathon in 1990 and 1995. Five pills of Arnica D30 or placebo were given morning and evening. Treatment started on the evening before the marathon and continued on day of the race and the three following days. The runners assessed muscular soreness on a visual analogue scale. Muscle enzymes, electrolytes and creatinine were measured before and after the marathon. RESULTS: Muscle soreness immediately after the marathon run was lower in the Arnica group than in the placebo group (P = 0.04). Cell damage measured by enzymes was similar in the Arnica and the placebo group. CONCLUSION: These pooled results suggest that Arnica D30 has a positive effect on muscle soreness after marathon running, but not on cell damage measured by enzymes.