Blackfoot disease is an endemic peripheral vascular disease found among people in a limited area on the southwest coast of Taiwan, where artesian well water has a high concentration of arsenic and was used since the turn of this century. This is an important public health problem and was noted by the authorities, who began improving the water supply in such communities in 1956. This enabled us to test the relationship between arsenic and malignant tumors using a specific exposed community. Study subjects were divided into four groups according to age (under or over 40 yr) and gender. Two methods were used for the estimation of the age-adjusted mortality rate ratios. First, using the first time interval (1971-1973) as the standard, the mortality rate ratio for all malignant tumors was estimated from this interval through to the last interval (1992-1994) using Poisson regression. Cancers that were found to be related to arsenic in previous reports, such as liver, lung, bladder, kidney, and skin cancers, were examined and other malignant tumors except these cancers were also assessed. The same calculations were performed for all of Chiayi and Tainan counties, excluding the study areas, which were used as the local reference, and for the general population of Taiwan, which was used as a national reference group. Second, mortality rate ratios for the study area were compared to the local and national reference for the same time intervals for each disease category. From our results, significantly declining trends for mortality rate ratios of all malignant tumors with 1971-1973 as the standard were found for the study areas, especially in females. A decrease of mortality rate ratios from malignant cancers, compared to the local or national references, was found in those aged over 40 yr for both sexes. The decreases are mainly due to a fall in internal and skin cancer mortality rates. In conclusion, our results suggest that the improvement of drinking water supply to eliminate arsenic exposure from artesian well water decreased the mortality incidence of arsenic-related cancers in blackfoot disease endemic communities.