OBJECTIVE While clinical observation has suggested an association between gastroesophageal reflux and laryngeal carcinoma, the nature of this relationship has yet to be defined. The purpose of this study is to determine the carcinogenic potential of acid and pepsin mixtures in the hamster cheek pouch animal model. METHODS A blinded intervention study. METHODS One hundred male Syrian hamsters aged approximately 5 weeks. METHODS A control group of 20 hamsters received application of the carcinogen 9,10-dimethyl-1,2-henzanthracene (DMBA) to their cheek pouch mucosa. One experimental group (n = 20) received applications of DMBA plus hydrochloric acid, and another (n = 20) received DMBA plus an acid and pepsin solution. Latency to squamous cell tumor production, size of tumors, and numbers of tumors were compared among groups. RESULTS Latency to tumor production and size of tumor were similar among groups, with both experimental and control groups developing tumors of comparable size after 12 weeks of chemical application. However, the number of tumors produced was significantly higher in the DMBA/acid and DMBA/acid/ pepsin groups than in the DMBA only group at 18 weeks, with 23, 27, and 10 tumors in these groups, respectively (P<.02). Likewise, a cumulative dysplasia score was different among groups at 18 weeks with the DMBA/acid and DMBA/acid/pepsin groups scoring higher degrees of dysplasia than the DMBA only group. CONCLUSIONS These results suggest that application of acid and acid/pepsin mixtures may promote experimental carcinogenesis in the hamster cheek pouch.