Background and aims Despite anticoagulation, usually with heparin, mortality for thromboembolic events in COVID-19 remains high. Clinical efficacy of heparin is due to its interaction with antithrombin (AT) that may be decreased in COVID-19. Therefore, we correlated AT levels with outcomes of COVID-19. Methods and results We recruited 49 consecutive patients hospitalized for COVID-19. AT levels were significantly lower in 16 non-survivors than in 33 survivors (72.2 ± 23.4 versus 94.6 ± 19.5%; p = 0.0010). A multivariate Cox regression analysis showed that low AT (levels below 80%) was a predictor of mortality (HR:3.97; 95%CI:1.38 to 11.43; p = 0.0103). BMI was the only variable that showed a significant difference between patients with low and those with normal AT levels (32.9 ± 7.9 versus 27.5 ± 5.9%; p = 0.0104). AT levels were significantly lower in obese patients than in subjects with normal weight or overweight (77.9 ± 26.9 versus 91.4 ± 26.9 versus 91.4 ± 17.1%; p = 0.025). An inverse correlation between AT levels and BMI was documented (r:-0.33; p = 0.0179). Conclusions Our data first suggest that AT is strongly associated with mortality in COVID-19. In addition, AT may be the link between obesity and a poorer prognosis in patients with COVID-19. Other studies should confirm whether AT may become a prognostic marker and a therapeutic target in COVID-19.