Fat embolism during total joint arthroplasty or intramedullary procedures is well documented and is infrequently fatal. Considerable morbidity is associated with fat embolism syndrome, and post operative cognitive dysfunction is frequently seen, yet the exact pathophysiology remains unclear. Intraoperative cerebral microemboli can be detected using transcranial Doppler ultrasound and moreover the presence of a patent foramen ovale (PFO) may be examined for using a validated technique employing this modality. Persistent patent foramen ovale may act as a conduit for embolic material to traverse from the venous to the systemic circulation and consequently affect cerebral function. We wished to 1) investigate the incidence of cerebral microembolisation during primary hybrid total hip arthroplasty and compare this with hip resurfacing, 2) examine the influence of patent foramen ovale on cerebral microembolisation and 3)assess the influence of cerebral microemboli on the outcome of patients undergoing these procedures. We prospectively compared 12 patients undergoing hip resurfacing with 12 patients undergoing hybrid total hip replacement (THR) for the incidence and load of intraoperative cerebral microemboli, using transcranial Doppler. All patients were tested for the presence of a patent foramen ovale using a validated technique. Outcome was assessed using the WOMAC, Harris Hip Score, Oxford Hip Score and EuroQoL quality of life measure. No patient in the hip resurfacing group demonstrated intra-operative cerebral microembolisation. Five patients in the THR group showed transcranial Doppler evidence of microemobli during the procedure. With the small numbers of patients available, there was not a significant difference in microemboli load between the groups (p = 0.09). There was no significant difference between the groups regarding the incidence of PFO (p = 0.78). There was no significant relationship between the incidence and total microemboli load and the incidence of PFO (p = 0.56). There was no significant difference in outcome at six months between patients who demonstrated microemboli and those who did not. The incidence of cerebral microembolisation during hip resurfacing appears to be very low. Although our study demonstrated cerebral microemboli in a significant proportion of patients undergoing primary hybrid THR, the numbers of microemboli were low and the presence of a patent foramen ovale did not influence microemboli incidence or load. Finally, patients who demonstrated cerebral microemboli did not have a worse outcome than patients who did not.