The acute-phase response (APR) is a frequent occurrence after infusion of zoledronic acid and is caused by activation of γδ T cells. Vitamin D receptor is expressed in immune cells, and vitamin D has immunomodulatory properties. The aim of this prospective study was to test the effect of vitamin D (cholecalciferol) on the incidence of APR and intensity of pain in women undergoing infusion of zoledronic acid for postmenopausal osteoporosis. 60 women were enrolled and randomized into two groups. At baseline, 30 women received an oral bolus of cholecalciferol (300,000 IU), while another 30 women received placebo. On day 5 both groups were treated with a single infusion of zoledronic acid (5 mg) and received a daily supplementation of calcium (1,000 mg) and vitamin D (800 IU). Patients were clinically evaluated and inflammatory markers were assayed before zoledronic acid administration and every 24 h for the following 2 days. The onset of APR has been defined by the occurrence of fever or at least one of the typical symptoms, such as musculoskeletal pain after zoledronic acid infusion. Intensity of pain was measured by a one-dimensional scale (0 = no pain, 10 = unbearable pain). APR developed in 66.6% of patients, with no significant difference between groups. The vitamin group experienced less musculoskeletal pain [median 1 (0-4) vs. 2 (1-8), P < 0.05] and exhibited lower inflammatory markers (P < 0.005 vs. placebo). Our data demonstrate that cholecalciferol at a dose of 300,000 IU reduces the intensity of musculoskeletal pain after infusion of zoledronic acid for postmenopausal osteoporosis.