The frequent association of asthma and paranasal sinusitis has been ascribed to a nasobronchial reflex, aspiration of sinus secretions, or enhanced β-adrenergic blockade. We investigated possible pulmonary aspiration in a pilot study (eight patients) and follow-up study (13 patients) by means of a radionuclide technique. In the pilot study, the aim was to demonstrate aspiration as well as visibility of the radionuclide in the thorax during a period of 24 hours. The radionuclide was initially placed bronchoscopically in the bronchial tree in four patients and was still clearly visible in the same position after 24 hours in three patients. Aspiration from the nasopharynx was unequivocally demonstrated in two of four patients with depressed consciousness. The follow-up study population consisted of four patients with maxillary sinusitis only and nine patients with sinusitis and asthma. The radionuclide was placed in a maxillary sinus during therapeutic puncture. In the patients with only sinusitis as well as patients with asthma and sinusitis the radionuclide could be demonstrated in the maxillary sinus, nasopharynx, esophagus, and lower gastrointestinal tract during a 24-hour period. However, no pulmonary aspiration of radionuclide could be demonstrated in any patient. We conclude that seeding of the lower airways by mucopurulent secretions is unlikely to account for coexistent pulmonary disease. The association is probably related to generalized mucosal disease affecting both upper and lower airways.