To confirm our impression that a high percentage of patients with amyotrophic lateral sclerosis are initially misdiagnosed, we reviewed records of 33 patients with a definitive diagnosis of amyotrophic lateral sclerosis seen over 10 years. Fourteen patients (43%) were initially misdiagnosed. Mean time to correct diagnosis was significantly greater for the misdiagnosed group (16.0 +/- 9.3 months) than for the rest of the patients (7.6 +/- 4.1 months). Two of three patients with an initial symptom of dyspnea were misdiagnosed. Three patients underwent laminectomies because of misdiagnosis. Age, stage of disease, and unusual presenting symptoms were not identified as causes of misdiagnosis. Most likely causes were physicians' failure to consider the diagnosis and lack of familiarity with the common clinical presentations of amyotrophic lateral sclerosis. Earlier diagnosis of amyotrophic lateral sclerosis may help prevent medical mismanagement and may benefit patients both medically and psychologically.