Primary hyperparathyroidism (PHPT) and vitamin D deficiency are two common diseases. Their combination seems also to be common and might influence the clinical and biological expression of PHPT. Forty two patients with PHPT in whom vitamin D (25OHD) was measured at diagnosis were studied retrospectively. Vitamin D deficiency (plasma 25OHD<10ng/ml) was observed in 40,5% of PHPT patients. Osteopenia and osteoporosis were more frequent (p=0,01) and PTH level was higher (567,9±697,0 µg/l vs 259,8±198,8 µg/l p=0.04) in patients with vitamin D deficiency. Low plasma 25OHD levels may contribute to the severity of PHPT. Measurement of 25OHD should be routinely performed in PHPT patients to better assess the severity of the disease and to consider possibly vitamin supplementation.