To evaluate dental arch relationships and dimensions, relative to an age matched noncleft sample, in Caucasian 3-year-old children with repaired unilateral cleft lip (UCL) or unilateral cleft lip and palate (UCLP). Prospective, cross-sectional, case-control study performed in Scotland, U.K. Eleven children with repaired unilateral cleft lip, 16 children with repaired unilateral cleft lip and palate, and 78 children as controls. Dental arch relationships and linear arch dimensions. Prevalence of Class III incisor relationship was 31.3% in children with unilateral cleft lip and palate compared with 9.1% in children with unilateral cleft lip. A buccal crossbite was present in 36% of children with unilateral cleft lip, compared with 75.6% of children with unilateral cleft lip and palate.Mean linear maxillary arch dimensions did not differ significantly between children with unilateral cleft lip and the controls. Except for second intermolar width, statistically significant differences existed in mean linear maxillary arch dimensions between the unilateral cleft lip and the unilateral cleft lip and palate groups; the mean linear maxillary arch dimensions were significantly greater in the control group than in the unilateral cleft lip and palate group. The mean cleft-affected anterior quadrant length appeared to be the arch dimension with the greatest power of discrimination among the three groups. There were no significant differences in mean linear mandibular arch dimensions among the three groups. Anterior crossbite was almost three times more common in the unilateral cleft lip and palate group than in the unilateral cleft lip group. Mean linear maxillary arch dimensions differed significantly between the unilateral cleft lip and palate group and the control group. There were no significant differences in mean linear maxillary arch dimensions between unilateral cleft lip and controls or between mean linear mandibular arch dimensions for unilateral cleft lip, unilateral cleft lip and palate, and controls.