Laryngomalacia is an abnormality of the voice box (larynx) that leads to the inward collapse of the airway when air is drawn into the lungs (inspiration). It usually becomes apparent at birth or shortly after birth. The most common symptom is noisy breathing (stridor) that is often worse when the infant is on his/her back or crying. In more severe cases, symptoms may include difficulty breathing with the chest pulling inward (retraction), poor weight gain from difficulty feeding, apnea, and cyanosis. The underlying cause of the condition is unknown. Most cases occur sporadically in people with no family history of the condition. In 90% of affected infants, laryngomalacia will resolve on its own by the time an infant is 18 to 20 months old. However, severe cases may require immediate medical treatment such as medication or surgery.

The development of symptoms of laryngomalacia may result from variable combinations of these factors in each infant. Regardless of the underlying causes(s), the symptoms are due to the cartilage being "floppy," and prolapsing (collapsing) over the larynx when inhaling. This in turn causes the noises generated when breathing.

Signs and symptoms of laryngomalacia typically begin at or shortly after birth, with the average age at presentation approximately 2 weeks. The condition is primarily characterized by noisy breathing (stridor) that may be worse when the baby is agitated, feeding, crying or sleeping on his/her back. However, in severe cases, inward pulling of the chest muscles (sternal or intercostal retraction), apnea, cyanosis or significant respiratory distress may be the initial symptom. Feeding difficulties are also common in infants with laryngomalacia and may result in poor weight gain or lead to failure to thrive. Many infants with laryngomalacia are also diagnosed with gastroesophageal reflux. The baby's cry is generally normal, unless laryngitis is present due to reflux.

Laryngomalacia has also been reported as being associated with various syndromes. In cases where these specific syndromes are inherited, a predisposition to being born with laryngomalacia may be present. However, even within a family, not all people affected with one of these syndromes will have the exact same signs and symptoms (including laryngomalacia). Syndromes that have been associated with laryngomalacia include diastrophic dysplasia, alopecia universalis congenital, XY gonadal dysgenesis, Costello syndrome, DiGeorge syndrome, and acrocallosal syndrome. The inheritance pattern depends upon the specific syndrome present.