We studied retrospectively patients with hyperparathyroidism after successful renal allotransplantation. Since 1972, 1119 transplantations have been performed in our department, and 534 patients survive with functioning grafts. Hyperparathyroidism requiring parathyroidectomy developed in 32 (5.9%). The frequency of interventions increased markedly after introduction of cyclosporine A treatment in our unit. The time between transplantation and parathyroidectomy was 22.5 months (SD 16.5, range 1-82 months). The age of the patients was 49.0 years (SD 10.5, range 17-63 years); the group consisted of 16 female and 16 male patients. All patients but two (no measurement performed) repeatedly exhibited high serum parathormone and calcium levels and therefore underwent surgery. In comparison to a control group, matched for time of transplantation, age, sex, and cause of renal failure, the patients with hyperparathyroidism had longer dialysis treatment (54.2 months, range 9-132 vs 26.9 months, range 1-72) and exhibited lower phosphate concentrations in the early posttransplantation period. Before surgery, serum chemistry was different for hyperparathyroid and control subjects: serum calcium 2.80 +/- 0.23 mmol/l vs 2.48 +/- 0.13 mmol/l and alkaline phosphatase 157.4 +/- 92.0 U/l vs 85.2 +/- 51.5, respectively. We did not see any influence of oral phosphate binders, calcium supplementation, or vitamin D treatment on the development of parathyroid gland hyperactivity during dialysis treatment. Serum creatinine concentration did not change after parathyroidectomy. In four patients, long-term calcium supplementation after surgery was necessary.