Laryngeal behavior in unilateral superior laryngeal nerve (SLN) paralysis was investigated in animal models and clinical cases. The occurrence of an oblique glottis caused by rotation of the posterior glottis to the paralytic side was the main focus of this study. The animal model study employed live dogs. When the SLN on one side was sectioned, spontaneous phonation did not cause a significantly oblique glottis. When the unaffected SLN was electrically stimulated during spontaneous phonation, an oblique glottis occurred. When the SLN was unilaterally stimulated during spontaneous phonation with both SLNs sectioned, a markedly oblique glottis occurred. In the clinical study, larynges of 17 patients with SLN paralysis were examined during a test task in which a low-pitched phonation was followed by a high-pitched phonation. The purpose of this task was to activate the unaffected cricothyroid muscle during the test phonation. Five patients could not perform the test task. The glottis obviously rotated in 9 patients, whereas no significant rotation of the glottis occurred in 3. One of the latter 3 had an incomplete paralysis, and the other 2 had marked scarring around the laryngeal framework. We conclude that a unilateral SLN paralysis causes a rotation of the posterior glottis to the paralytic side when the unaffected cricothyroid muscle is markedly activated. The test consisting of low-pitched phonation followed by high-pitched phonation is a relatively simple and accurate diagnostic procedure for unilateral SLN paralysis.