In a limited area on the southwest coast of Taiwan, where artesian well water with a high concentration of arsenic has been used for more than 60 years, a high prevalence of chronic arsenicism has been observed in recent years. The total population of this "endemic" area is approximately 100,000. A general survey of 40,421 inhabitants and follow-up of 1,108 patients with blackfoot disease were made. Blackfoot disease, so-termed locally, is a peripheral vascular disorder resulting in gangrene of the extremities, especially the feet. The overall prevalence rates for skin cancer was 10.6 per 1000, and for blackfoot disease 8.9 per 1000. Generally speaking, the prevalence increased steadily with age in both diseases. The prevalence rates for skin cancer and blackfoot disease increased with the arsenic content of well water, i.e., the higher the arsenic content, the more patients with skin cancer and blackfoot disease. A dose-response relationship between blackfoot disease and the duration of water intake was also noted. Furthermore, the degree of permanent impairment of function in the patient was directly related to duration of intake of arsenical water and to duration of such intake at the time of onset. The most common cause of death in the patients with skin cancer and blackfoot disease was carcinoma of various sites. The 5-year survival rate after the onset of blackfoot disease was 76.3%; the 10-year survival rate was 63.3% and 15-year survival rate, 52.2%. The 50% survival point was 16 years after onset of the disease.