The palatal forms in 20 infants with a complete unilateral cleft lip, and palate (12 with a Hotz palate and 8 without, selected at random) were studied from birth until 18 months of age. Using techniques developed previously, the degree of curvature in the palate and the magnitude of migration of the maxillary segments were measured three-dimensionally. Furthermore, using a newly developed method to approximate a set of the points on the alveolar ridge to a circle in a plane, the form of the alveolar arch was evaluated. Results from the group with a Hotz plate revealed that the plate possesses four effects not seen in the group without a Hotz plate. The size of the palate was larger, and the sagittal gap between the two segments of the maxilla was smaller. These results suggest that the appliance could stimulate the growth of the segments and could prevent collapse of the maxillary arch from the force of lip closure. Third, the steepness of the segments toward the nasal cavity was smaller, possibly occurring because the appliance prevents tongue intrusion into the cleft. Fourth, the magnitude of migration of the lesser segment toward the cleft edge of the major segment was larger. This result suggests that the appliance could guide the growth of the maxillary segments to narrow the cleft width until 18 months of age.