Objectives: (a) To determine 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 analyze the association of vitD levels with both the clinical manifestations and the immunological profile of patients with primary APS. (c) To perform a meta-analysis evaluating potential differences in serum vitD levels between APS and controls as well as the frequency of vitD deficiency in APS patients. Methods: Retrospective study including 74 patients with primary APS, 54 with positive antiphospholipid (aPL) serology not meeting clinical criteria for APS, and 215 healthy controls. We considered 30 and 10ng/ml as the thresholds for vitD insufficiency and deficiency, respectively. Meta-analysis included four case-control studies (325 primary APS patients and 507 controls) and was conducted by fitting random effects models and checked for heterogeneity. Results: Median serum vitD levels were similar in the three groups: 21ng/ml in primary APS, 25ng/ml in the aPL-positive group, and 21ng/ml in controls (p = 0.115). However, we found differences in the PTH levels, being 40.4 ± 24.9pg/ml in APS, 34.1 ± 18.2pg/ml in aPL serology, and 23.4 ±12.6pg/ml in healthy controls (p < 0.001). Regarding vitD deficiency, we found significant differences across the groups: 16.2% in APS, 11.1% in patients with positive serology, and 3.7% in controls (p = 0.001). There was a trend for the presence of thrombotic events in patients with vitD deficiency (38.9% vs 19.1%, p = 0.071). The meta-analysis confirmed that the combined mean difference in serum vitD levels between APS and controls was -3.605 (p < 0.001) and that APS patients had an increased frequency of vitD deficiency, with an OR = 3.06 (95% CI: 2.12-4.43, p < 0.001). Conclusions: APS patients show higher frequency of vitD deficiency than the healthy individuals. The meta-analysis study, including three cohorts and ours, suggests that APS patients have significantly lower serum vitD levels and higher frequency of vitD deficiency than controls.