Parathyroidectomy (PTX) is one of the treatments of choice for secondary hyperparathyroidism in chronic dialysis patients. Due to the large increase in long-term dialysis patients, hyperparathyroidism is becoming a common clinical problem. Several studies on PTX have reported various surgical procedures, but limited information is available on the incidence and risk factors of the surgery. The Okinawa Dialysis Study (OKIDS) registry is a community-based dialysis registry. It covers the entire area of Okinawa from when the use of chronic dialysis began in 1971. By the end of 1990, a total of 1,986 chronic dialysis patients were registered and 128 of these had undergone PTX by the end of 1993. The cumulative incidence of PTX was 4.3 in DM and 15.2 in non-DM per 1,000 patient-years. About half of the PTX patients underwent the surgery within 10 years of dialysis. By logistic analysis, the risk of PTX was seen to increase significantly with the duration of dialysis, P < 0.0001. Other clinical variables such as sex, age at the start of dialysis and the presence of diabetes mellitus were not significant predictors of PTX. The probability of PTX increased linearly with the duration of dialysis (r = 0.83, p < 0.0001). After the introduction of active vitamin D in 1981, the probability of PTX was significantly decreased (p < 0.05) compared to the pre-vitamin D period ('71-'80). With prolongation of the duration of dialysis, the risk of PTX increased steadily and was estimated to be 10 percent in 10 years and 20 percent in 20 years. Other uremic factors determining a pathological transformation of parathyroid tissue from reactive to autonomous growth remained to be investigated.