Nasal obstruction is known to cause abnormal ventilation during sleep in infants, but its effects on breathing and oxygenation during sleep in adults are unknown. However, in adults, obstruction of the nose by nasal packing has been shown to cause hypoxia, and on occasion, hypercarbia and sudden death. We have investigated the pattern of ventilation and the level of oxygenation during sleep in seven patients who had nasal packs after nasal polypectomy or septoplasty. Using standard polysomnographic techniques, we monitored chest wall motion, nasal and oral airflow, and arterial oxygen saturation and sleep stages. Nasal packing either caused or worsened sleep-disordered breathing in all patients and significantly increased the number, duration, and frequency of episodes for the group as a whole. Several patients also had a greatly increased number and severity of episodes of nocturnal oxygen desaturation. This study shows that obstruction of the nose by packing causes marked alterations in breathing during sleep in adults.