AbstractA phantom material to represent lungs radiologically is described. The material consists of an open-cell, plastic foam the density of which may be varied over a wide range by adjustment of the amount of blowing agent used. Details of the chemical procedure are given. The process is simple and requires a maximum processing temperature of only 115°C and the material can be shaped easily by the use of moulds. The effective atomic number of the plastic was calculated andmeasured and details are given. The unexpanded material has a density of 1·06 gcm−3, an effective atomic number of 7·55 and 3·42 electrons per gram. Tissue-equivalence is considered satisfactory and is compared with data for other tissue-equivalent substances. A review of other materials which have been used to simulate lungs is given.