Background Acute massive or submassive pulmonary embolism (PE) has high mortality, but the clinical course according to the location of onset (ie, in-hospital or out-of-hospital) is unknown. Methods and Results In the present study 56 consecutive patients with acute massive or submassive PE were studied retrospectively and a comparison made of the clinical characteristics, and outcomes between in-hospital onset (Group A) and out-of-hospital onset (Group B). Patients in Group A (n=28) had more frequent comorbidities with hemodynamic instability (54% vs 4%, p<0.0001) and temporary risk factors (93% vs 11%, p<0.0001), whereas patients in Group B (n=28) had a longer duration of symptoms (median: 5.5 days vs 0.5 day; p<0.0001), and had higher systolic pulmonary artery pressure (63±17 mmHg vs 46±12 mmHg, p=0.0006). Although in-hospital mortality did not differ between the 2 groups, the recurrence rate was higher in Group B (23% vs 0%, p=0.03). Conclusions Patients who had in-hospital onset of PE had mostly temporary risk factors, unstable hemodynamics and a lower recurrence rate compared with the cases of out-of-hospital onset. In cases of in-hospital onset, prompt diagnosis and suitable treatment is needed to prevent fatalities and cases of out-of-hospital onset should be followed carefully for recurrence. (Circ J 2004; 68: 988 - 992)