The aim of this study was to describe the course of parathyroid hormone (PTH) and serum calcium after surgery for primary hyperparathyroidism, and to evaluate the usefulness of daily measurement of these parameters. PTH and serum calcium were daily monitored during the early postoperative period in 45 patients where a parathyroid adenoma was removed. PTH showed a sharp drop to a very low level already on the first postoperative day, whereafter a rapid recovery began. Serum calcium decreased more slowly: on the average, the lowest calcium level was seen on the second day. A majority of the patients were temporarily hypocalcemic, apparently to various degrees. After two weeks 38 patients showed a normal calcemia, while six patients remained slightly hypocalcemic. It is concluded that after removal of a parathyroid adenoma an abrupt fall of PTH precedes the decrease of serum calcium. The first day's PTH level is a reliable indicator of the success of the intervention and it should be a major point in the postoperative biochemical follow-up. Repeated measurements of serum calcium are useful, in contrast with the daily dosage of PTH which can be omitted for economical reasons.