The descriptive epidemiology of cancer of three upper respiratory tract sites (lip; mouth and pharynx, excluding nasopharynx; and nose, sinus and nasopharynx) was explored using data from 1972-1976 from the population-based cancer registry covering Los Angeles County, California. In addition, the proportion of Los Angeles cases at each site which are explained by tobacco use was estimated using attributable risks calculated from previously published case-control studies. Causes of nontobacco-related cases are considered. Lip cancer was found to be a disease predominantly of white males with a male/female ratio of 9.8 for the lower lip and of 1.0 for the upper lip. Tobacco use is responsible for 60% of male lip cancers, and incidence rates of lip cancer after subtraction of the proportion of the incidence due to tobacco use were more than three times greater in men than in women. A hypothesis is proposed that an important etiologic factor is exposure to sun, wind and cold which results in chapping; this exposure is most intense for the lower lip; and the use of lip coverings in women protects their lips both from the sun and from chapping. Cigarette smoking was responsible for 51% of mouth and pharynx cancer in men and women in Los Angeles County, and after subtraction of the proportion of the incidence due to smoking, rates in men and women were similar. Factors such as poor dentition and nutritional deficiencies may help explain demographic characteristics such as the higher rates of mouth and pharynx cancer among men and women in the lower social classes and the secular decline in incidence rates. Cancers of the nose, sinus and nasopharynx were found to occur among workers in a wide range of occupations and industries which involved inhalation of dust, fumes and vapors. High incidence rates of nasopharynx cancer were observed among Chinese in Los Angeles and high rates of sinus cancer among Spanish-surnamed residents. Cancers at these sites appear to be related to the action of carcinogens on tissue chronically irritated by repeated upper respiratory infections or allergic responses. In addition racial differences in the physical structure of the nasal passage may influence where inhaled particles are deposited.