Aim of the study was to evaluate the effect of infant orthopaedics (IO) on maxillary dimensions in infants with unilateral cleft lip and palate (UCLP). The study design was a prospective two-arm randomised controlled trial in parallel with three participating academic Cleft Palate Centres. Treatment was assigned by means of a computerised balanced allocation method. One group (IO+) wore passive maxillary plates during the first year of life, the other group (IO-) did not. Maxillary casts, made at birth, and at 15, 24, 48, 78 wk were digitised three-dimensionally. Before lip closure alveolar, midpalatal and posterior cleft width reduced significantly more in IO(+) than in IO(-). After lip closure, the alveolar cleft width reduced significantly more in IO(-). Until soft palate closure the slope of the palatal vault flattened significantly by IO. It is concluded that IO only has a temporary effect on maxillary arch dimensions that does not last beyond surgical soft palate closure. Therefore, infant orthopaedics as a tool to improve maxillary arch form could be abandoned. However, other outcome variables like facial and dental appearance, speech outcome, and cost-effectiveness need to be investigated further in order to assess the comprehensive effect of infant orthopaedics.