Congenital nasolacrimal obstruction is a common disorder in infants that results in persistent tearing and may lead to infections, such as dacryocystitis, orbital cellulitis, and bacterial conjunctivitis. The true incidence of this disorder in healthy newborns remains controversial. The most frequently quoted number of 6% comes from a study of 200 consecutive live births in the 1940s in which nasolacrimal patency was assessed by the presence or absence of discharge on compression of the lacrimal sac. Estimates from other studies, which often use different criteria for diagnosis, vary considerably from 1.2% to 30%. The incidence of the disorder is higher in children who have craniofacial disorders and Down's syndrome. This article reviews the causes and treatment of congenital nasolacrimal obstruction.