Adenotonsillar hypertrophy is common among children, but it can lead to serious complications if left untreated. Among the well-known complications are obstructive sleep apnea syndrome, growth failure, cor pulmonale, and hypertension. One complication of adenotonsillar hypertrophy that has not been previously reported in the English-language literature is transient cortical blindness. We describe such a case, which occurred in a 6-year-old boy who presented with a sudden loss of vision and subsequent unconsciousness. He had experienced hypercapnia and was resuscitated via endotracheal tube ventilation. Laboratory and radiologic assessments found no pathology except for extremely enlarged adenoid tissue. Once the patient was stabilized, an urgent adenotonsillectomy was performed. The patient recovered well, and his vision and respiratory symptoms resolved. Severe hypertrophy of the adenoid tissue can cause hypercapnia and acidosis secondary to upper airway obstruction. The possibility of adenoid hypertrophy and hypercapnia should be kept in mind in cases of transient cortical blindness. Aggressive treatment, including early intubation and adenoidectomy, may lead to a rapid resolution of symptoms.