Context: Impairments of muscle function and strength in patients with primary hyperparathyroidism (PHPT) are rarely addressed although decreased muscle function may contribute to increased fracture risk. Objective: We aimed to assess changes in muscle strength, muscle function, postural stability, quality of life (QoL), and well-being during treatment with vitamin D or placebo before and after parathyroidectomy (PTX) in PHPT patients. Design: Randomized placebo controlled trial. Patients: We included 46 PHPT patients, mean age 58 (range 29-77) years and 35 (76%) were women. Interventions: Daily treatment with 70 microgram (2,800 IU) cholecalciferol or placebo for 52 weeks. Treatment was administered 26 weeks prior to PTX and continued for 26 weeks after PTX. Main outcome measures: Changes in QoL and measures of muscle strength and function. Results: Preoperatively, 25-hydroxyvitamin D (25OHD) increased significantly (50 to 94 nmol/L) compared with placebo (57 to 52 nmol/L). We did not measure any beneficial effects of supplementation with vitamin D compared with placebo regarding well-being, QoL, postural stability, muscle strength, or function. In all patients, we measured marked improvements in QoL, well-being (p<0.01), muscle strength in the knee flexion and extension (p<0.001), and muscle function tests (p<0.01) after surgical cure. Postural stability improved during standing with eyes closed (p<0.05), but decreased with eyes open (p<0.05). Conclusions: Patients with PHPT and 25OHD levels around 50 nmol/L did not benefit from vitamin D supplementation concerning muscle strength, muscle function, postural stability, well-being, or quality of life. Independent of preoperative 25OHD levels, PTX improved these parameters.