Arsenic has been well documented as the major risk factor for blackfoot disease (BFD), a unique peripheral vascular disease that was endemic to the southwestern coast of Taiwan, where residents consumed artesian well water containing high levels of arsenic for more than 50 yr. Chronic arsenic exposure was also reported to be associated with mortality attributed to prostate cancer in a dose-response relationship. A tap-water supply system was implemented in the early 1960s in the BFD-endemic areas in Taiwan. Artesian well water was no longer used for drinking and cooking after the mid-1970s. The objective of this study was to determine whether prostate cancer mortality decreased after the improvement of drinking-water supply system through elimination of arsenic ingestion from artesian well water. Standardized mortality ratios (SMRs) for prostate cancer were calculated for the BFD-endemic area for the years 1971-2006. Results showed that mortality attributed to prostate cancer declined gradually after the improvement of drinking-water supply system. Based on the reversibility criterion, the association between arsenic exposure and development of prostate cancer is likely to be causal.