&NA;Quantitative information on soil water retention is in demand in hydrology, agrometeorology, agronomy, contaminant transport, and other soil‐related disciplines of earth and environmental sciences. Soil aggregate composition is an important characteristic of soil structure and, as such, is expected to affect soil water retention. Our objective was to determine whether and how aggregate size distributions affect soil water retention from saturation to the wilting point. Soil properties were studied for samples of Podzoluvisols, Planosols, Chernozems, Fluvisols, Calcisols, and Gleysols. Cumulative particle size and aggregate size distributions were used in regression trees to determine what fractions provide the least heterogeneous groups of samples as determined from values of water retention at matric potentials of −10, −33, and −1500 kPa as well as from van Genuchten parameters of the water retention curves, all on a gravimetric basis. Soil aggregate composition provided important grouping parameters for almost all water retention characteristics in this work. Contents of either small aggregates (<0.25 mm, <0.5 mm, <1 mm) or large aggregates (>7 mm, >10 mm) were the grouping variables in most cases for parameters of the van Genuchten equation. No aggregate size distribution parameters were included in the regression tree for the water content at −10 kPa, whereas contents of aggregates of medium sizes do affect water content at −33 and −1500 kPa. Aggregate size distribution parameters can be useful in estimating parameters of soil water retention from other soil properties. (Soil Science 2003;168:223‐233)