Objective: Animal studies describe cardiovascular collapse (CVC; hypotension or reoccurrence of cardiac arrest) after return of spontaneous circulation (ROSC) from cardiopulmonary arrest. Few studies describe CVC in humans. This study aimed to determine the occurrence of CVC in human out-of-hospital cardiopulmonary arrest (OHCA). Methods: Using observational data from a site of the Resuscitation Outcomes Consortium, the study analysed treated, non-traumatic OHCA achieving initial ROSC. CVC was defined as post-ROSC hypotension (systolic blood pressure ≤80 mm Hg), post-ROSC administration of epinephrine, vasopressin or dopamine, or post-ROSC recurrent cardiac arrest. The time period from initial ROSC to emergency department (ED) arrival was measured. The prevalence of and elapsed time to post-ROSC CVC was determined, censoring cases at the point of ED arrival and comparing clinical characteristics between CVC and non-CVC cases. Results: Of 1081 treated OHCA, ROSC occurred in 58 (5%; 95% CI 4% to 7%). CVC occurred in three cases of 58 ROSC (5%; 95% CI 1% to 14%), all due to recurrent cardiac arrest. The median ROSC to ED arrival time was 6 min (IQR 3-13 min). ROSC to CVC times were 1, 2 and 8 min. Patient sex, age, initial ECG rhythm, endotracheal intubation, bystander cardiopulmonary resuscitation and bystander automated external defibrillation were similar between CVC and non-CVC cases (p=0.11-1.00). Conclusions: In this series of treated OHCA, only a small fraction of patients experienced CVC after ROSC.