In a double-blind, randomized, multicentre clinical study, the efficacy and tolerance of a herbal medicine product, Harpadol (6 capsules/day, each containing 435 mg of powdered cryoground powder Harpagophytum procumbens), was compared with diacerhein 100 mg/day in the treatment, for 4 months, of 122 patients suffering from osteoarthritis of the knee and hip. Assessments of pain and functional disability were made on a 10 cm horizontal visual analogue scale; severity of osteoarthritis was evaluated by Lequesne's index. Spontaneous pain showed a significant improvement during the course of the study and there was no difference in the efficacy of the two treatments. Similarly, there was a progressive and significant reduction in the Lequesne functional index and no statistical difference was found between Harpadol and diacerhein. At completion of the study, patients taking Harpadol were using significantly less NSAIDs and antalgic drugs. The frequency of adverse events was significantly lower in the Harpadol group. The most frequent event reported was diarrhea, occurring in 8.1% and 26.7% of Harpadol and diacerhein patients respectively. The global tolerance assessment by patients at the end of treatment favoured Harpadol. The results of this study demonstrate that Harpadol is comparable in efficacy and superior in safety to diacerhein.