Although the two disease concepts have very different histories, many previous studies have mixed conversion disorder and somatization disorder and none has made direct comparison between them. The authors applied DSM-III criteria to inpatient and outpatient medical records and attempted to follow 98 patients who met criteria for somatization disorder or conversion disorder. Five of these patients died 4 years later and, of those who survived, 70 (75.3%) were given follow-up interviews by a rater blind to baseline diagnosis. The 32 patients with a baseline diagnosis of conversion disorder were significantly less likely than the 38 patients with somatization disorder to be given the same diagnosis at follow-up. Six of the conversion disorder patients were given follow-up diagnoses of somatization disorder and, in four other cases, subsequent developments revealed medical explanations for the presenting complaint. Of the two baseline diagnoses, somatization disorder predicted substantially more impairment in a variety of domains.