The development of the dental arch is well designed for adaptive and compensatory growth. In this study, the relationship between the sleep position and dental arch development was investigated. A group of 42 infants with unilateral complete cleft lip and palate with either prone (16) or supine (26) sleep position were seen in the craniofacial center. All infants were less than 1 month of age at the initial visit. Dental impressions of the maxillary arch were taken at the initial visit and just before cheiloplasty. Ten arch dimensions were measured in each dental cast and the longitudinal change in each dimension was compared between the prone sleep group and the supine sleep group. Statistically significant changes were detected in the growth rate of the following dimensions: intercanine width, intertuberosity width, alveolar cleft width, anterior cleft width, and posterior cleft width. This study indicated that sleep position affected maxillary arch development. Infants sleeping in the prone sleep position tended to have narrower arch width and cleft width.