1. Posterior choanal atresia is an uncommon condition, but it is not as rare as the scant radiologic literature would indicate. 2. The findings in 19 cases of unilateral and bilateral choanal atresia are presented, with the emphasis on symptoms, diagnosis, and treatment. It is stressed that the newborn infant is an obligate nasal breather and that oral breathing is an acquired ability. 3. A new clinical classification of bilateral choanal atresia, based on the ease with which the neonate learns both to breathe orally and to co-ordinate mouth breathing and sucking, is proposed as a replacement for the traditional anatomic classification. 4. Bilateral choanal atresia must be ruled out in any neonate with respiratory distress. Failure to quickly establish the diagnosis and institute immediate therapy may result in preventable death from asphyxia. 5. Unilateral choanal atresia is usually a benign condition and elective therapy suffices.