The value of Hounsfield unit (HU) assigned to a particular Requests for reprints should be addressed to: Mr G. J. Oettlb, area is a mean value for the tissue contained in the volume of Department of Surgery, Medical School, University of the the area. If the area contains structures of varying density, Witwatersrand, 7 York Road, Parktown, Johannesburg 2193, each structure contributes proportionally to the HU. Very South Africa. highor low-density objects contribute corresponding density values, which when averaged with the rest of the structures in the volume result in abnormally high or low HU being assigned to the entire area. This is termed the partial volume effect. The effect may be minimized by using finer slice thicknesses to limit the volume included in each slice (Wegener, 1983). Although in this patient the CT slices had been only 3 mm thick, partial volume effect resulted in poor visualization of the denser foreign body lying as it did within the air track. Since in the normal course of events pockets of air are absorbed within days, a persistent air track should alert the clinician to the possibility of a foreign body. A detailed search for radiologically subtle foreign objects should always follow. Awareness of this may help avoid the pitfall created by the partial volume artefact, and encourage an active search using all available modalities to detect any inconspicuous object.