The study analyses the changes occurring in the maxillary arches of 30 infants with complete unilateral clefts of the lip and palate following presurgical maxillary orthopaedic treatment. 15 similar cases not receiving such treatment were used as controls together with 30 normal subjects. Photocopies of models of the arches at birth and 4 months were analyzed by computer and the area of palatel tissue measured using stereophotogrammetry. In the presurgical cases, the palatal cleft was significantly narrowed due to the transverse and antero-posterior dimensions of the arches increasing less than in the controls. The narrowing of the cleft was particularly marked in the alveolar region because the presurgical treatment minimised the antero-lateral (outward) rotation of the lesser segment which otherwise would have occurred. Somewhat surprisingly, the study showed that presurgical treatment tended to inhibit growth of palatal tissue and it was concluded therefore that the narrowing of the defect was due mainly to the growth pattern of the maxillary arch being modified by the appliances.