Spontaneous pneumomediastinum is a rare entity in children, especially in young infants. We report the case of a 4-month-old infant with a massive spontaneous pneumomediastinum caused by acute viral bronchiolitis. Treatment including bed rest, codeine for its antitussive action, and nitrogen washout resolved the pneumomediastinum within 3 days. In the literature, cases of pneumomediastinum in very young infants are exceptional. To our knowledge, codeine has never been used in this situation. Nitrogen washout is also rarely used because of poorly demonstrated efficacy.