['We compared temporal trends in serum 25-hydroxyvitamin D and parathyroid hormone (PTH) in two primary hyperparathyroidism (PHPT) cohorts recruited 20 years apart. The prevalence of 25-hydroxyvitamin D levels <20 and <30 ng/mL declined by 30-50 %, respectively, and was accompanied by lower PTH. In the older cohort, higher PTH may be due to lower 25-hydroxyvitamin D.', 'Vitamin D deficiency may exacerbate PHPT. Whether there have been temporal trends in 25-hydroxyvitamin D (25OHD) levels in PHPT is unclear. The prevalence of low vitamin D levels (25OHD <20 and <30 ng/mL) and associated biochemical and bone mineral density (BMD) profiles were assessed in two PHPT cohorts recruited over 20 years apart.', 'This is a cross-sectional comparison of serum 25OHD levels, calciotropic hormones, and BMD between two PHPT cohorts recruited at the same hospital: the "old" (N\u2009=\u2009103) and "new" (N\u2009=\u2009100) cohorts were enrolled between 1984 and 1991 and between 2010 and 2014, respectively.', 'Mean 25OHD levels were 26 % higher in the new cohort (23\u2009±\u200910 vs. 29\u2009±\u200910 ng/mL, p\u2009<\u20090.0001). Levels of 25OHD <20 and <30 ng/mL declined from 46 and 82 %, respectively, to 19 and 54 % (both p\u2009<\u20090.0001). Supplemental vitamin D use was common in the new (64 %) but not the old cohort (0 %). The new cohort demonstrated 33 % lower serum PTH levels (p\u2009<\u20090.0001). Neither serum nor urine calcium differed. BMD was higher in the new cohort at all skeletal sites (all p\u2009<\u20090.001).'] Do low vitamin D levels have become less common in primary hyperparathyroidism?