The present study was designed to test the hypothesis that undermining of the soft tissues on the surface of the maxilla at the time of lip repair in unilateral cleft lip, alveolus, and palate results in more severe craniofacial growth aberrations than lip repair alone. Sixty-seven purebred beagles were used in this experiment. The animals were divided into four groups: two control groups (unoperated and unrepaired) and two experimental groups (lip repair without undermining and lip repair with undermining). Lip pressures were monitored in all groups. Significantly higher lip pressures were observed in animals with soft-tissue undermining. Cephalometric measurements were analyzed using univariate and multivariate techniques. The results of this study indicate that lip repair performed with soft-tissue undermining results in more severe craniofacial growth aberrations than lip repair performed alone.