There are no data about the optimal supplementation therapy in HIV-infected patients with vitamin D (25OHD) deficiency. The aim of this study was to assess the effect of an oral monthly dose of 16,000 IU calcidiol. We performed a longitudinal cohort study of 365 HIV-infected patients (24 % females) was with sequential determinations of 25OHD, serum parathyroid hormone (PTH), calcium, and alkaline phosphatase. The efficacy and safety of supplementation in 123 patients were compared against dietary and sun exposure advice. Overall, mean baseline 25OHD levels were 19.1 ng/ml (IQR 12-23.6), 63 % of patients had 25OHD deficiency and 27 % secondary hyperparathyroidism. After a median time of 9.3 months (95.61 patients-year on-treatment), 25OHD levels increased in comparison with non-supplemented patients (+16.4 vs. +3.2 ng/ml; p < 0.01), decreasing the rate of 25OHD deficiency (from 84 to 24 %), and decreasing serum PTH (-4.9 pg/ml) and the rate of secondary hyperparathyroidism (from 43 to 31 %; p < 0.001). This improvement was observed irrespective of HIV/HCV coinfection or the use of efavirenz. In a regression analysis, adjusting by seasonality, a lower baseline 25OHD was associated with persistence of deficiency (relative risk, RR 1.07; 95 % CI 1.03-1.1; p < 0.001), whereas calcidiol supplementation was the only factor associated with significant improvement (RR 0.38; 95 % CI 0.12-0.46; p < 0.001). This monthly dose showed no clinical toxicity, and no patient had 25OHD levels above 100 ng/ml, nor hypercalcemia. The use of monthly calcidiol is safe, easy to take, and largely effective to improve vitamin D deficiency and secondary hyperparathyroidism in HIV-infected patients.