Abstract Two different methods for the analysis of data produced by a Coulter counter were used to obtain rates of calcium oxalate crystal growth and aggregation occuring in a seeded crystallization system: (1) crystal growth was expressed as the increase in crystal volume, and aggregation as the decrease in crystal numbers observed by the Coulter counter; (2) crystal growth was expressed as the linear increase in crystal diameter, calculated using a computer model which, when calculating extents of aggregation, takes account of any crystals moving into and out of the field of view of the instrument. Data from experiments using different concentrations of seed crystal were analysed by these two methods. Expressing crystal growth as the increase in volume showed growth rates to be directly proportional to the total surface of seed crystals present, while expressing the same growth as the linear increase in crystal diameter showed growth rates to be independent of this variable. This difference in expression of experimental data became important when urine was included in the experimental system, and varying degrees of crystal aggregation affected the amount of surface area available for crystal growth. Expressing growth as the increase in crystal volume, and aggregation as the uncorrected decrease in crystal number, resulted in overestimation of inhibitory activities of urine towards crystal growth and aggregation by 60% and 40%, respectively. Calculation of crystal growth rates from the linear increase in crystal diameter, and aggregation rates from data corrected for the crystals moving through the field of view of the particle counter, are essential for the valid interpretation of such data.