We determined activities of adenosine deaminase (ADA), 5′ nucleotidase (5NT), xanthine oxidase (XO), superoxide dismutase (CuZn SOD), and catalase (CAT) enzymes in 15 human laryngeal tissues with-differentiated squamous cell carcinomas, in 15 corresponding tumor-free adjacent tissues and in 7 normal laryngeal tissues. We found lower ADA and 5NT and higher XO, CuZn SOD, and CAT activities in cancerous tissues than those in corresponding noncancerous ones. In the correlation analysis, we established one positive intercorrelation, which was between ADA activities of tumor tissues and noncancerous adjacent tissues. We also found some significant intracorrelations between enzyme activities of the tissues, all of which were positive in cancerous ones.