Sixty children (3-11 years) were evaluated to determine variations in naso- and oropharyngeal dimensions associated with tonsil and adenoid hypertrophy. The subjects were grouped according to tonsil size and a clinical history of chronic upper airway obstruction. Intraoperative measurements included oropharyngeal diameter, length of the hard and soft palates, width and arch of the hard palate, nasopharyngeal volume, as well as tonsil and adenoid weights and volumes. A significantly larger oropharyngeal diameter was found in children with small, non-obstructing tonsils (P less than 0.01). Children with large, non-obstructing tonsils had a similar oropharyngeal diameter to those children with large, obstructing tonsils. However, tonsil volume, not weight, was increased in the children with large obstructing tonsils as compared to those with large non-obstructing tonsils and small non-obstructing tonsils (P less than 0.04). A shorter soft palate was associated with larger, obstructing tonsils (P less than 0.004). The length of the hard palate was similar in all patients, however, a trend towards a higher arched palate was seen in patients with larger, obstructing tonsils. The distance from the soft palate to the posterior pharyngeal wall was greater in obstructed patients with adenotonsillar hypertrophy (P less than 0.003). In patients requiring adenoidectomy, the nasopharyngeal volume prior to adenoidectomy was significantly smaller in patients with obstructive symptoms (P less than 0.001). Postadenoidectomy, no significant difference was found in the nasopharynx volume amongst all subjects. These data indicate that subtle differences in oropharyngeal dimensions exist which along with increased lymphoid tissue volume, lead to the development of obstructive symptoms. Etiologic considerations are discussed.