This review outlines briefly the history and present status of the problem of carcinogenic, teratogenic and mutagenic effects of arsenic. Discrepancies between clinical observations and positive results of epidemiological studies and the experimental induction of cancer by arsenic are discussed. The present knowledge of the mechanism of teratogenic and mutagenic effects of arsenic is analyzed. The growing importance of arsenic as an environmental pollutant is demonstrated. Continuation of throughly organized epidemiological studies in regions with excessive arsenic exposure of the population and standardization of an epidemiological approach to this problem on an international basis are recommended. New approaches in experimental studies of the carcinogenicity of arsenic in combination with other known or suspected carcinogens are recommended as well.