Long-term exposure to inorganic arsenic is related to increased risk of cancer in the lung, skin, bladder, and, possibly, other sites. However, little is known about the consequences of developmental exposures in regard to cancer risk. During early summer in 1955, mass arsenic poisoning of infants occurred in the western part of Japan because of contaminated milk powder. Okayama Prefecture was most severely affected. We examined whether the affected birth cohorts in this prefecture experienced increased cancer mortality.