Cleft lip is a congenital slit in the upper lip due to the incomplete fusion of the maxillary and intermaxillary prominences. This disorder may be unilateral or bilateral, and is easily repaired through plastic surgery.

Cleft can be caused by genetic predisposition, but environmental factors can also play a big role. Substance abuse or use of certain pharmaceutical drugs (e.g., Accutane) by the pregnant mother has been demonstrated to increase incidence of cleft lip.