PURPOSE: Pulmonary embolism (PE) can compromise gases exchanges of patients admitted to respiratory wards. The objectives of this observational study were to identify radiological and arterial blood gases (ABG) differences in PEs documented by computed tomography pulmonary angiogram (CTPA) in Covid-19 pneumonia (C-19) vs cancer patients (CP). METHODS: Fifty-one patients diagnosed with PE were evaluated, 18 CP and 33 C-19. Clinical, ABG and radiological parameters were evaluated. Jamovi software was used for statistics. RESULTS: Wells score and D-dimer were both significantly higher in CP compared to C-19 cohort. DVT prevalence was significantly higher in CP patients. Alveolar-arterial O2 gradient (ΔA-aO2) was significantly higher in C-19 compared to CP cohort. PE associated to Covid-19 pneumonia was often bilateral and required a higher need of respiratory supports as High flow nasal cannula (HFNC), Continuous positive airway pressure (CPAP) and Non-invasive ventilation (NIV). In both cohorts, standard PaO2 and standard P/F were significantly lower than PaO2 and P/F. Multivariable linear regression analysis showed that in C-19 cohort PESI score and A-aO2 were independently and positively associated with PE severity. DISCUSSION: These findings show that Wells score and D-dimer may underestimate PE's risk in C-19 patients. P/F may underestimate the severity of respiratory failure compared to standard P/F. A-aO2 may be a useful tool for prediction of PE severity.