A case report of a congenital midline sinus of the upper lip in a 5-year-old girl is presented. The patient complained of intermittent swelling around the frenulum of the upper lip. The sinus was located on the midline of the philtrum just below the base of the columella. Excision of the sinus tract via an intraoral approach gave a satisfactory result. Microscopic examination of the resected sinus revealed it to be lined by stratified squamous epithelium with sebaceous glands and hair follicles. Fifteen other cases have been reported in the English literature and are reviewed. There are two postulates that can account for the formation of the upper lip sinus based on two major theories of the development of the face: the fusion theory and the merging theory. However, the etiology of this rare congenital sinus remains obscure.