In summary, evidence from human populations has shown that smokeless tobacco users have risks of cancer several times higher than that of nonsmokers. Smokeless tobacco is very strongly related to cancers of the cheek and gums, locations typically in direct contact with the tobacco. The association is evident in studies in the US and Scandinavia and also in Asia, where the chewing of quids containing tobacco and other ingredients is strongly related to the region's high rates of oral cancer. Reports on health effects date back to the 18th century and the evidence is consistent, with most investigators reporting positive findings. Dose-response relationships--increasing risk with increasing use--are also evident in several studies. Finally, preliminary work on cancers in other anatomic sites suggests that smokeless tobacco may also be related to other upper digestive tract cancers. With the exception of one group of medical scientists who described the relationship as only an "association," expert consensus conferences and committees in both the US and France have agreed that the evidence from clinical, epidemiologic, and carcinogenesis studies supports a causal relationship between the use of smokeless tobacco and oral cancer in humans. These expert groups include the International Agency for Research on Cancer (1984), the National Institutes of Health Consensus Conference on the Health Implications of Smokeless Tobacco Use (1986), and the Surgeon General's Advisory Committee on the Health Consequences of Using Smokeless Tobacco (1986).(ABSTRACT TRUNCATED AT 250 WORDS)