Functional gastrointestinal disorders are a variable combination of chronic or recurrent medically unexplained gastrointestinal symptoms. They can be conceptualized within the biopsychosocial model of illness as a dysregulation of the brain-gut axis and its relationships with psychosocial variables (psychopathology, health care seeking, life events, somatosensory amplification). Psychopathology may be undetected with the standard psychiatric criteria, particularly at a subclinical level. Using the new classification of the Diagnostic Criteria for Psychosomatic Research (DCPR) for assessing psychosocial components of somatic illnesses, psychosomatic syndromes were found at a prevalence of 2.5 times greater than DSM-IV diagnoses. In particular, alexithymia, persistent somatization, functional somatic symptoms secondary to a psychiatric disorder, and demoralization were the most prevalent syndromes. Furthermore, psychosomatic severity (as measured with the presence of more than one DCPR conditions) strongly predicted the treatment outcome in patients with functional gastrointestinal disorders. In particular, alexithymia and persistent somatization were independent predictors of unimprovement (and health anxiety of improvement) after 6 months of treatment as usual, after controlling for gastrointestinal symptoms at baseline. DCPR may therefore be suggested as a reliable assessment instrument for psychological conditions that are relevant for psychosomatic practice and research settings but that are not included in the DSM-IV.