Background & Aims: Unexplained, multi-system somatic symptoms and syndromes, the hallmark features of somatization, are prevalent in patients with functional gastrointestinal disorders (FGIDs). We studied outpatients attending a gastroenterology clinic to see whether current somatic symptom burdens (a somatization state measure) or number of prior functional diagnoses (a somatization trait measure) could predict the presence of an FGID over structural gastrointestinal disease, and whether the predictive value was dependent on comorbid depression or anxiety disorders. Methods: Clinical data from 327 consecutive new referrals to an outpatient gastroenterology practice were reviewed, 187 with an FGID and 140 with a structural illness. Somatization state and trait were measured by using self-reported current symptoms and functional diagnoses recorded in the medical history, respectively. Psychiatric comorbidity (depression or anxiety disorders) was extracted from chart review. Results: FGID subjects endorsed more somatization state symptoms, had more somatization trait diagnoses, and had greater likelihood of psychiatric comorbidity (P < .001 for each). Logistic regression analysis adjusting for age and sex differences showed that each of these features independently predicted the likelihood of an FGID over structural disease (P < .05 for each). When high ratings on the somatization measures were present together with psychiatric comorbidity, the positive predictive value exceeded 0.95. Conclusions: Higher burdens of either current somatic symptoms or functional diagnoses in the medical history are strong predictors of an FGID in outpatients presenting with gastrointestinal complaints. The mechanism is not solely dependent on a relationship with affective disorders, which independently predicts FGID, at least in part, through another path.