Purpose:To study COX 2 protein expression and its significance in familial adenomatous polyposis (FAP) and rectal cancer tissues. Methods: Flow cytometry was used to detect COX 2 protein expression in surgical specimens from 12 normal rectal mucosa tissues, 19 FAP tissues, 7 transformed FAP tissues and 36 rectal cancer tissues.Results:The COX 2 protein expression was negative in the 12 normal rectal mucosa tissues ( FI=0.67?0.28). The COX 2 protein expression was positive in 18 (94.74%) of the 19 FAP tissues (FI=1.97?0.36),the 7(100%) transformed FAP tissues(FI=2.02?0.24) and the 36(100%) rectal cancer tissues (FI=2.14?0.31). The COX 2 protein and positive expression rates showed that there were obvious differences between normal rectal mucosa tissues and FAP tissues, transformed FAP tissues, rectal cancer tissues( P 0.05). Conclusions:It is suggested that the detection of COX 2 protein expression can have important significance for predicting adenomatous polyposis carceration and chemo interfering carceration progress. [