A Technical Panel of the U.S. Environmental Protection Agency's Risk Assessment Forum has studied three special issues regarding certain health effects, particularly skin cancer, associated with arsenic ingestion: the validity of the Tseng et al. (1968) study of skin cancer in Taiwan and its use for dose-response assessment in the U.S. population; the interpretation and use of skin lesions reported as arsenic-induced skin cancers in that study; and the role of arsenic as an essential nutrient in the human diet. The Technical Panel also reviewed auxiliary information on genotoxicity, metabolism, and other factors that might suggest the most appropriate approach to dose-response assessment. In brief summary, the analysis shows a causal relationship between ingestion exposure to arsenic and an increased risk of skin cancer in the Taiwan population. This leads to a classification of arsenic as a Group A human carcinogen under EPA's cancer guidelines. However, in the absence of fully persuasive evidence for any of the possible mechanisms of arsenic-induced carcinogenesis, a generalized multistage model that is linear at low doses was used to place an upper bound on the expected human cancer dose-response.