In this investigation the microdialysis technique was used to study the concentrations of lactate in Achilles tendons with painful chronic tendinosis and in normal pain-free tendons. In four patients (mean age 40.7 years) with a painful thickening localized at the 2-6 cm level in the Achilles tendon (chronic Achilles tendinosis) and in five controls (mean age 37.2 years) with normal Achilles tendons the local concentrations of lactate were registered under resting conditions. All tendons were examined using ultrasonography. In the tendons with tendinosis the painful thickening corresponded to a widened tendon and structural tendinosis changes. Normal tendons showed no widening and a normal structure. A standard microdialysis catheter was inserted into the Achilles tendon under local anesthesia. Samplings were done every 15 min during a 4 h period. The results showed significantly higher mean concentrations of lactate in tendons with tendinosis compared to normal tendons (2.15 mmol/l vs. 1.14 mmol/l). The lactate concentrations in the tendons with tendinosis were stable, and approximately twofold higher than in the normal tendons during the whole 4 h investigation period. In conclusion, the higher concentrations of lactate in Achilles tendons with painful tendinosis indicate that there are anaerobic conditions in the area with tendinosis. The importance of this finding for the pathogenesis and pain mechanisms in this chronic condition needs to be further investigated.