Riancho-Zarrabeitia L, et al. - A retrospective cohort study and meta-analysis are carried out: a) to ascertain serum 25-OH vitamin D (vitD) levels in primary antiphospholipid syndrome (APS) and to compare them with patients with positive antiphospholipid serology who do not meet clinical criteria for APS, and with healthy controls. b) To examine the relationship of vitD levels with both the clinical manifestations and the immunological profile of patients with primary APS. c) To perform a meta-analysis assessing potential differences in serum vitD levels between APS and controls and also the frequency of vit D deficiency in APS patients. The outcome of this study suggests that the APS patients demonstrate a higher frequency of vitD deficiency than the healthy individuals. The meta-analysis study, including three cohorts and theirs, recommends that APS patients have significantly lower serum vitD levels and higher frequency of vitD deficiency than controls.