OBJECTIVE The purpose of this study was to compare dental arch size, mesiodistal and buccolingual crown widths, and crown shapes between 2 groups of children, 1 with anterior crowding in both dental arches and another with anterior spacing in both dental arches. METHODS Sixty-one sets (27 crowded and 34 spaced arches) of dental casts of normal occlusions were examined. RESULTS For both lower and upper arches, crowded arches had statistically significantly smaller arch widths than did the spaced arches. The buccolingual width of the upper second primary molar of the crowded arches was statistically significantly larger than that of the spaced arches. However, there were little significant differences between crowded and spaced arches in arch length, mesiodistal crown width, and crown shape. CONCLUSIONS It can be concluded that the factor that determines whether a primary dental arch is crowded is the absolute width of the arch.