Background. Although vitamin D has been reported to be useful in the treatment of patients with secondary hyperparathyroidism, it is not effective in some of them. The goal of this study was to see whether a relationship could be found between maxacalcitol responsiveness and parathyroid gland size. Methods. Parathyroid gland size was measured by ultrasonography in 25 patients with secondary hyperparathyroidism serum intact parathyroid hormone (PTH) >300 pg/ml, 58.1 ± 2.8 years old, 15 males and 10 females, who were treated with maxacalcitol. Patients were divided into two groups according to the mean value of the maximum diameter of the glands: group S with a diameter <11.0 mm and group L with a diameter ≥11.0 mm. Between the two groups there were no significant differences in serum intact PTH, calcium or phosphate level or duration of haemodialysis. Results. Mean (± SE) maximal diameter of detectable parathyroid glands was 11.0 ± 0.7 mm before treatment. At 4–24 weeks after administration of maxacalcitol, intact PTH concentrations decreased significantly in group S (from 546 ± 39 to 266 ± 34 pg/ml at 24 weeks; P < 0.01), but did not significantly change in group L (from 481 ± 39 to 403 ± 49 pg/ml at 24 weeks). At 24 weeks after maxacalcitol administration, the number of detectable parathyroid glands was significantly decreased in group S (from 2.2 ± 0.3 to 1.8 ± 0.4; P < 0.05), but not in group L. Serum calcium increased significantly in group L (from 9.6 ± 0.2 to 10.2 ± 0.3 mg/dl; P < 0.05), but not in group S. There was a significant correlation between reduction in PTH and parathyroid gland size (r = −0.42, P < 0.05). Conclusions. These results indicate that the responsiveness to maxacalcitol therapy of secondary hyperparathyroidism is dependent on parathyroid gland size and that the simple measurement of maximum parathyroid gland diameter by ultrasonography may be useful for predicting responsiveness to maxacalcitol treatment.