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 45 years, a high prevalence of chronic arsenic poisoning has been observed in recent years. The total population of this endemic area, which is defined by the presence of both patients and contaminated wells, is approximately 100,000. A general survey of 40,421 inhabitants (19,269 males and 21,152 females) of the area was made. The overall prevalence rates for skin cancer, hyperpigmentation, and keratosis were 10.6, 183.5, and 71.0/1000, respectively. The male-to-female ratio was 2.9: 1 for skin cancer and 1.1: 1 for hyperpigmentation and keratosis, respectively. Generally speaking, the prevalence increased steadily with age in all three conditions, though there was a decline in age groups above 69 in females with cancer or hyperpigmentation. The prevalence rate for skin cancer showed an ascending gradient according to the arsenic content of the well water, i.e., the higher the arsenic content, the more patients with skin cancer. The results were the same for hyperpigmentation and keratosis. Blackfoot disease, so-termed locally, a peripheral vascular disorder resulting in gangrene of the extremities, especially of the feet, had an overall prevalence rate of 8.9/1000. A dose-response relationship between this disease and the amount of arsenic in the well water was similar to that observed for skin cancer. The association of Blackfoot disease with hyperpigmentation, keratosis, and skin cancer was significantly higher than expected. The causal relationship between Blackfoot disease and chronic arsenicism is discussed.