BACKGROUND: It is unclear which factors explain the high co-morbidity between functional dyspepsia (FD) and other functional somatic syndromes. The aim of this study is to investigate the association between gastric sensorimotor function, psychosocial factors and 'somatization' on the one hand, and co-morbid irritable bowel syndrome (IBS) and chronic fatigue (CF)-like symptoms on the other, in FD. METHODS: In 259 tertiary care FD patients, we studied gastric sensorimotor function with barostat (sensitivity, accommodation). We measured psychosocial factors (abuse history, alexithymia, trait anxiety, depression, panic disorder) and 'somatization' using self-report questionnaires, and presence of IBS and CF-like symptoms. Hierarchical multiple logistic regression was used to determine which of these factors were independently associated with co-morbid IBS and CF-like symptoms, including testing of potential mediator effects. KEY RESULTS: Co-morbid IBS or CF-like symptoms respectively were found in 142 (56.8%) and 102 (39.4%) patients; both co-morbidities were not significantly associated (P=0.27). Gastric accommodation (ß=0.003, P=0.04) and 'somatization' (ß=0.17, P= 0.0003) were independent risk factors for IBS (c=0.74, P<0.0001); the effect of adult abuse (ß=0.72, P=0.20) was mediated by 'somatization'. Depression (ß=0.16, P=0.008) and 'somatization' (ß=0.18, P=0.004) were overlapping risk factors for CF-like symptoms (c=0.83, P<0.0001); the effects of alexithymia and lifetime abuse were mediated by depression and 'somatization', respectively. CONCLUSIONS & INFERENCES: 'Somatization' is a common risk factor for co-morbid IBS and CF-like symptoms in FD and mediates the effect of abuse. Gastric sensorimotor function and depression are specific risk factors for co-morbid IBS and CF-like symptoms, respectively.