Chest radiographs taken under ideal conditions and reviewed by a radiologist experienced in looking for pulmonary oedema are an accurate technique for detecting early oedema, differentiating between congestion and oedema, monitoring the increases in oedema up to the stage of alveolar flooding and in determining the effects of CPPV on an oedematous lung. However, ideal conditions do not exist in the I.C.U. Our radiologist had the advantage of a fast exposure, a normal film for comparison in the sequence and only one lesion, conditions unlikely to occur in the I.C.U. It becomes apparent that it is important to obtain the best possible image in these complicated patients. Films in the erect position, for vascular distribution, as well as fast exposures with a high MAS factor give the sharp detail required to best assess lung changes. Serial studies of the chest are a requirement of the I.C.U. patient and it is helpful if comparable films are obtained by using a fixed target-to-film distance in association with the short exposure factors. If this care is taken, chest radiographs are the most accurate non-invasive technique for detecting pulmonary oedema.