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The method determines absolute body volume by measuring the pressure/volume behavior of air inside a closed chamber containing the subject. If polyurethane foam is placed inside the chamber, the air behaves isothermally during the measurement, which means that Boyle’s law can be applied. The characteristics of the polyurethane foam are such that it also eliminates interference from water vapor and heat production. No detectable deviations were observed between this plethysmometric procedure and the underwater weighing method (r = 0.99 and p < 0.00001), using either objects or human volunteers. It can be concluded that the new method of measuring body volume offers promising possibilities for future research in body composition. Applications in clinical practice and epidemiological surveys are envisaged.