This study was designed to evaluate the clinical usefulness of 3-dimensional computed tomography (CT) laryngography for the determination of subglottic extension in laryngeal cancer and for the involvement of the pyriform sinus apex in hypopharyngeal cancer. Twenty-one patients with laryngeal cancer and 19 patients with hypopharyngeal cancer underwent spiral CT during quiet breathing, /E/ phonation, Valsalva's maneuver, and modified Valsalva's maneuver. Three-dimensional volume rendering with CT data was performed on a workstation. Three-dimensional CT laryngography was compared to laryngoscopy, axial CT, endoscopy, and/or histopathologic findings. Three-dimensional CT laryngography was useful for showing subglottic extension in laryngeal cancer and involvement of pyriform sinus apex in hypopharyngeal cancer. Three-dimensional CT laryngography with quiet breathing was most informative for the determination of subglottic extension, whereas 3-dimensional CT laryngography with modified Valsalva's maneuver was most informative for the diagnosis of pyriform sinus apex involvement. In patients with vocal cord palsy, the contour of the vocal cord was the same during all 4 maneuvers. Three-dimensional CT laryngography during quiet breathing, /E/ phonation, Valsalva's maneuver, and modified Valsalva's maneuver may be useful in assessing laryngeal and hypopharyngeal cancers. This technique may provide important information not provided by conventional CT images.