The muscle strength of the quadriceps muscle was measured in a computerized modified Cybex II muscle dynamometer. The method used is easy and has an error of greater than 5%. A marked increase in muscular strength was found after successful surgery for primary hyperparathyroidism (PHPT) with normalization of serum calcium levels. The results support the opinion that all patients with PHPT, with or without muscle weakness, should be recommended an operation if no serious contra-indications exist.