In order to define the role of Single Photon Emission Computed Tomography (SPECT) in the diagnosis of pulmonary embolus; SPECT and Planar ventilation and perfusion lung studies were performed consecutively on eleven patients referred with suspected embolus. Three patients were shown to have 'high probability' ventilation perfusion mismatches. SPECT imaging allowed segmental localisation of the perfusion defect and revealed additional defects not seen on planar scans. SPECT lung study was performed with minimal technical difficulty and was well tolerated by all patients studied. SPECT is likely to become the method of choice for investigating patients referred with suspected pulmonary embolus.