"Eating disorder NOS" is the most common eating disorder encountered in outpatient settings yet it has been neglected. The aim of this study was to describe the characteristics of eating disorder NOS, establish its severity, and determine whether its high relative prevalence might be due to the inclusion of cases closely resembling anorexia nervosa or bulimia nervosa. One hundred and seventy consecutive patients with an eating disorder were assessed using standardised instruments. Operational DSM-IV diagnoses were made and eating disorder NOS cases were compared with bulimia nervosa cases. Diagnostic criteria were then adjusted to determine the impact on the prevalence of eating disorder NOS. Cases of eating disorder NOS comprised 60.0% of the sample. These cases closely resembled the cases of bulimia nervosa in the nature, duration and severity of their psychopathology. Few could be reclassified as cases of anorexia nervosa or bulimia nervosa. The findings indicate that eating disorder NOS is common, severe and persistent. Most cases are "mixed" in character and not subthreshold forms of anorexia nervosa or bulimia nervosa. It is proposed that in DSM-V the clinical state (or states) currently embraced by the diagnosis eating disorder NOS be reclassified as one or more specific forms of eating disorder.