The concentration of substrate expressed as hypoxanthine capable of reacting with xanthine oxidase to release superoxide free radicals (O2-) was measured in control and Dupuytren's contracture palmar fascia. In Dupuytren's contracture palmar fascia the concentration of hypoxanthine was six times that of control and was greatest in "nodular" areas. Xanthine oxidase activity was also detected in Dupuytren's contracture palmar fascia. These results suggest a greater potential for hypoxanthine-xanthine oxidase generated oxygen free radical formation in Dupuytren's contracture than in control palmar fascia. Production of free radicals may be an important factor in the pathogenesis of Dupuytren's contracture. The benefit of allopurinol in the management of Dupuytren's contracture and other fibrotic conditions may thus be explained, as allopurinol binds to xanthine oxidase and prevents release of free radicals.