In a prospective study of 20 patients with median lip fissure, pedigrees were obtained. A hereditary predisposition for weakness in the first branchial arch fusion seems to exist. In some persons, more often in males than in females, this weakness eventually leads to development of a median lip fissure. The fissure becomes symptomatic when it is infected. In 10 patients the fissure was excised; in the other 10 patients excision was done in combination with a Z-plasty of the center of the lesion. The prognosis with the latter operation was superior to that with the former.