This study was conducted to examine the rates of somatization disorder (SD) in the chronic fatigue syndrome (CFS) relative to other fatiguing illness groups.It further addressed the arbitrary nature of the judgments made in assigning psychiatric vs. physical etiology to symptoms in controversial illnesses such as CFS. Patients with CFS (N = 42), multiple sclerosis (MS) (N = 18), and depression (N = 21), were compared with healthy individuals (N = 32) on a structured psychiatric interview. The SD section of the Diagnostic Interview Schedule (DIS) III-R was reanalyzed using different criteria sets to diagnose SD. All subjects received a thorough medical history, physical examination, and DIS interview. CFS patients received diagnostic laboratory testing to rule out other causes of fatigue. This study revealed that changing the attribution of SD symptoms from psychiatric to physical dramatically affected the rates of diagnosing SD in the CFS group. Both the CFS and depressed subjects endorsed a higher percentage of SD symptoms than either the MS or healthy groups, but very few met the strict DSM-III-R criteria for SD. The present study illustrates that the terminology used to interpret the symptoms (ie, psychiatric or physical) will determine which category CFS falls into. The diagnosis of SD is of limited use in populations in which the etiology of the illness has not been established.