Monarch Disease Ontology entry MONDO_0011331 (congenital chylothorax) is a rare, potentially life-threatening neonatal condition characterized by the accumulation of chyle within the pleural space leading to respiratory distress, malnutrition and immunological compromise, either immediately after birth or within the first few weeks of life. Congenital chylothorax is the most common cause of pleural effusion in neonates; it can occur primarily due to developmental anomalies of the lymphatic duct or can be associated with chromosomal anomalies (e.g. Noonan syndrome, Turner syndrome and Down syndrome), hydrops fetalis, mediastinal neuroblastoma and other congenital malformations.