Plain abdominal radiographs were studied in 280 children aged between one day and fourteen years to assess the age-related width of the extra-peritoneal fat. At the same time the limitations of assessing the size of the right side of the liver, the lower pole of the spleen and the psoas shadow were investigated. A significant rise in the evaluation of this method was only obtained after the sixth year and this was 86% in relation to the right lobe of the liver and 36% in relation to the lower pole of the spleen. From this age onwards, the preperitoneal fat is demonstrable in 94% of cases. The investigation suggests that from the sixth year onwards, the absence of the extra peritoneal fat line is an indirect indicator that there is intra-peritoneal fluid or a retro-peritoneal tumour. Eck