The mortality experience of all pensioners from a copper smelter who were aged 65 or over between 1949 and 1973 has been studied. An index of arsenic exposure was developed for all operations in the plant for 1973. This was applied to all individuals studied so that a comparative measure could be made of each individual's working-life exposure to arsenic. The overall mortality of this cohort was 12.2% higher than for males living in the same area, of the same ages, and in the same time periods. The excess mortality was due chiefly to respiratory cancer where mortality was three times the expected. Because other contaminants were present in the atmosphere, it is not certain that arsenic was entirely responsible for the respiratory cancer observed, yet a close association with arsenic seems highly likely.