Objective: To describe apnoea test (AT) and ancillary study performance for brain death (BD) determination among patients undergoing short-term mechanical circulatory support (MCS) devices, including extracorporeal membrane oxygenation (ECMO) and intra-aortic balloon pump (IABP). Methods: We retrospectively analysed data regarding use of AT and ancillary study in consecutive adult patients who were diagnosed with BD while on MCS devices (including ECMO and IABP) over a 10-year period. Results: Out of 140 patients, eight were on MCS devices at the time of BD (four ECMO, two ECMO and IABP, two IABP). The most common aetiology of BD was hypoxic ischaemic brain injury (6/8, 75%). In four patients (50%), the AT was not attempted because of haemodynamic instability and ECMO; in the remaining four (50%), both AT and ancillary studies were used. In three patients on ECMO, AT was performed by reducing the ECMO sweep flow rate to a range 0.5–2.7 L/min in order to achieve hypercarbia. One patient underwent AT while on IABP which was complicated by hypotension. All patients underwent ancillary tests, most commonly transcranial Doppler ultrasonography (TCD) (7/8, 88%); among those, cerebral circulatory arrest was confirmed in six of seven patients (86%), all of whom had left ventricular ejection fracture (LVEF) ≥20% and/or were supported with IABP. Conclusions: There are multiple uncertainties regarding BD diagnosis while on MCS, prompting the need for ancillary studies in most patients. Our study shows that TCD can be used to support BD diagnosis in patients on ECMO who have sufficient cardiac contractility and/or IABP to produce pulsatile flow. TCD use in ECMO patients low LVEF needs further study.