Harmonisation of the assessment of pesticide leaching to groundwater for EU registration is desirable to minimise confusion in the decision-making process at EU level. Recently, the FOCUS groundwater scenarios have been developed for three chromatographic models (PEARL, PELMO and PRZM) to increase this harmonisation. This study investigates the role of dispersion parameterisation in explaining the cause of the differences in pesticide leaching calculated by these models. PEARL describes dispersion via a physical parameter, ie the dispersion length. PELMO and PRZM simulate dispersion via a numerical procedure which generates an effective dispersion length equal to 0.5 times the thickness of the numerical compartments. The hypothesis was tested that the difference in the dispersion length input parameter (ie 5 cm for PEARL and about 2.5 cm for PELMO and PRZM) is a major cause of the difference in calculated leaching. It was tested whether results of PEARL calculations with a dispersion length of 2.5 cm corresponded much better to results of PELMO or PRZM than results of PEARL calculations with a dispersion length of 5 cm. This was done by calculations for one substance and all nine FOCUS scenarios and by calculations for a range of substances and two FOCUS scenarios (Châteaudun and Sevilla). All calculations were for winter wheat and an application at 1 day before emergence. Both tests showed that reduction of the dispersion length from 5 to 2.5 cm in PEARL led to a much better correspondence between PEARL and either PELMO or PRZM. Hence the hypothesis was supported. It is likely that harmonisation of the dispersion length in the FOCUS groundwater scenarios would reduce the differences in calculated leaching between PEARL and PELMO or PRZM considerably for part of these scenarios. Published in 2004 for SCI by John Wiley & Sons, Ltd.