OBJECTIVE: Patients with coronavirus disease 2019 (COVID-19) present coagulation abnormalities and thromboembolic events that resemble antiphospholipid syndrome (APS). This work has aimed to study the prevalence of APS-related antigens, antibodies, and immune complexes in patients with COVID-19 and their association with clinical events. METHODS: A prospective study was conducted on 474 adults with severe acute respiratory syndrome coronavirus 2 infection hospitalized in two Spanish university hospitals. Patients were evaluated for classic and extra-criteria antiphospholipid antibodies (aPLs), immunoglobulin G (IgG)/immunoglobulin M (IgM) anticardiolipin, IgG/IgM/immunoglobulin A (IgA) anti-ß2-glicoprotein-I (aß2GPI), IgG/IgM antiphosphatidylserine/prothrombin (aPS/PT), the immune complex of IgA aß2GPI (IgA-aß2GPI), bounded to ß2-glicoprotein-1 (ß2GPI) and ß2GPI levels soon after COVID-19 diagnosis and were followed-up until medical discharge or death. RESULTS: Prevalence of aPLs in patients with COVID-19 was as follows: classic aPLs, 5.8%; aPS/PT, 4.6%; IgA-aß2GPI, 15%; and any aPL, 21%. When patients were compared with individuals of a control group of a similar age, the only significant difference found was the higher prevalence of IgA-aß2GPI (odds ratio: 2.31; 95% confidence interval: 1.16-4.09). No significant differences were observed in survival, thrombosis, or ventilatory failure in aPL-positive versus aPL-negative patients. ß2GPI median levels were much lower in patients with COVID-19 (15.9 mg/l) than in blood donors (168.8 mg/l; P < 0.001). Only 3.5% of patients with COVID-19 had normal levels of ß2GPI (>85 mg/l). Low levels of ß2GPI were significantly associated with ventilatory failure (P = 0.026). CONCLUSION: ß2GPI levels were much lower in patients with COVID-19 than in healthy people. Low ß2GPI-levels were associated with ventilatory failure. No differences were observed in the COVID-19 evolution between aPL-positive and aPL-negative patients. Functional ß2GPI deficiency could trigger a clinical process similar to that seen in APS but in the absence of aPLs.