Arsenic has been well documented as the major risk factor for blackfoot disease (BFD), a unique peripheral vascular disease (PVD) that was endemic in the Southwestern coast of Taiwan, where residents had used high-arsenic artesian well water for more than 50 years. Long-term arsenic exposure has also been reported to associated with lung cancer mortality in a dose-response relationship. A tap-water supply system was implemented in the early 1960s in the BFD-endemic areas. Artesian well water was no longer used for drinking and cooking after the mid-1970s. The objective of this study is to examine whether lung cancer mortality decreased after the improvement of drinking water supply system through elimination of arsenic exposure from artesian well water. Standardized mortality ratios (SMRs) for lung cancer were calculated for the BFD-endemic area for the years 1971-2000. The study results show that mortality from lung cancer declined gradually after the improvement of drinking water supply system to eliminate arsenic exposure from artesian well water. Based on the reversibility criterion, the association between arsenic exposure and lung cancer mortality is likely to be causal.