Secondary hyperparathyroidism (2HPT) is characterized by an abnormal threshold for suppression of parathyroid hormone (PTH) secretion by serum ionized Ca (ICa). It is therefore critical to examine the threshold of PTH secretion in chronic renal failure patients in relation to the physiopathological conditions and the severity of 2HPT. The effect of 1,25-dihidroxy-22-oxavitamin D(3) (22-oxacalcitriol, OCT) on parathyroid gland function was investigated in six haemodialysis patients with 2HPT. OCT was administered three times a week for 26 consecutive weeks. The maximum serum PTH (PTHmax), the minimum serum PTH (PTHmin), and ICa concentration required to inhibit 50% of PTHmax were estimated based on the model formula of the sigmoid curve describing the relationship between ICa and intact-PTH levels. The sigmoid ICa-PTH curves displayed a downward shift at 12 and 26 weeks of OCT treatment. Parathyroid gland function, as reflected by both PTHmax and PTHmin, decreased over time. A steep slope was found at 12 and 26 weeks, compared with that at the start of OCT treatment. There were no marked changes in the set point of calcium. Hypercalcaemia and elevated creatine phosphokinase, probably due to OCT therapy, were observed during the study period. In view of the findings that the sigmoid ICa-PTH curve displayed a downward shift, that both PTHmax and PTHmin decreased, and that functional mass of the parathyroid gland was reduced, OCT appears to be useful in ameliorating parathyroid gland function, contributing to the management of 2HPT.