Interviews with 866 patients with cancer of the oral cavity and pharynx and 1249 controls of similar age and sex from the general population in four areas of the United States revealed increased risks associated with the regular use of mouthwash. Risks of oral cancer were elevated by 40% among male and 60% among female mouthwash users, after adjusting for tobacco and alcohol consumption. Risks among both sexes generally increased in proportion to duration and frequency of mouthwash use. The increased risks were confined to users of mouthwash high in alcohol content, consistent with the elevated risks associated with drinking alcoholic beverages. Except for a higher prevalence of leukoplakia among cases, little relationship was found with oral or dental conditions, although denture wearing was reported more often by patients with cancer of the gums. These findings, together with other studies, provide further incentive for clarifying the association between mouthwash use and oral cancer.