A foot plethysmograph is described and evaluated. It is intended for recording changes in foot volume when the instrument has to be removed from the foot between measurements to make physical activity possible. The mean sensitivity was ±0·5 ml and the mean experimental error 1·6 ml or 0·16% of the mean foot volume (n = 37). The accuracy of the method is thus considerably higher than that of previously reported methods for corresponding applications. In addition, it is inexpensive and can easily be used outside the laboratory.