Abstract A new method is presented of determining the cumulative size distributions of aerosols from data obtained with a multistage cascade impactor and high efficiency backing filter. Given the capture efficiency-particle diameter characteristics of the impactor stages and the density of the particle material, the experimenter need only record the relative masses of deposit on the stage of the impactor and filter after aerosol sampling. With an N-stage impactor, the aerosol is theoretically approximated by a mixture of N + 1 monodisperse aerosols in proportions determined from the experimental data, and then the equivalent cumulative size distribution of the mixture is synthesised. The calculation involved is considerable, but is conveniently performed by computer, making the method suitable for routine use. The application to May's cascade impactor has been examined in detail, employing a computer program which gives up to twenty-four pairs of values of diameter and cumulative frequency. Tests have been made with a wide variety of model size distributions. These show that the median diameter and the interquartile diameters are determined to within 5 per cent and 15 per cent respectively, provided that the experimental data used are accurate. Furthermore, the shape of the distribution curve is clearly shown, even with bimodal distributions. The method is a significant improvement on May's E.D.S. method and is worthy of use, especially for comparative purposes.