Dental study casts on 35 patients with unilateral cleft lip and palate (UCLP) were evaluated at the late primary and early mixed dentition stages for the presence of significant (more than one tooth) posterior crossbites. Using this crossbite criterion patients were divided into "acceptable" and "unacceptable" groups. Eleven arch dimensions on the presurgical dental casts were measured to see if any, either singly or in combination, could be used to predict the later occurrence of significant crossbite. No single measurement of infant arch dimensions significantly differed between children with and without crossbite. Data from 26 patients were used for discriminant function analysis. This produced an equation correctly predicting the presence or absence of posterior crossbites in 88% of cases in the primary and 91% in the mixed dentitions.