In the period from 1985 to 1992, 136 athletes underwent 148 surgical exploration of Achilles (98 operations in 90 patients) and patellar (50 operations in 46 patients) tendons for persistent pain. The morphology and the biochemistry of tendon samples taken at operation were investigated. A marked increase of glycosaminoglycans, especially chondroitin sulphate, was present. The complete alleviation of pain and the return to full activity were obtained in 79% of the athletes, while the remaining 21% were not able to return to their sports activities. Generally, successful outcome after surgery was experience by younger patients who had tendon pain of short duration--which was correlated to a lesser degree of tendon degeneration.