Irritable bowel syndrome (IBS) is a common and potentially disabling gastrointestinal (GI) disorder that is subject to strong psychological influences particularly among more severe IBS patients. Little is known about the role of actionable lifestyle factors (e.g., obesity) that influence the trajectory of other chronic diseases. This study examined the associations between obesity and different aspects of illness experience among more severe IBS patients. We hypothesized that Body Mass Index (BMI) would positively correlate with worse health outcomes including more severe IBS symptoms, extraintestinal complaints, and emotional distress. At pretreatment baseline in a National Institutes of Health (NIH)-funded behavioral trial, 448 Rome-diagnosed IBS patients (MAGE = 41; MBMI = 26, Female = 8%) were administered a test battery that included a variety of clinical (IBS symptom severity, fear of GI symptoms, BMI, etc.), and sociodemographic (e.g. age, etc.) variables. BMI was positively and significantly correlated with somatization (unexplained somatic complaints) but not IBS symptom severity or emotional distress. A series of moderated multiple regression analyses showed that the associations between BMI and somatization were moderated by the interaction between BMI and age, and fear of GI symptoms. Older patients with higher BMI reported higher levels of somatization and patients who were more fearful of GI symptoms were more likely to experience somatization if they also had a high BMI. These data highlight the relationship between lifestyle factors and extraintestinal symptoms among more severe IBS patients and the impact of both sociodemographic (age) and psychosocial (fear of GI symptoms) factors on this relationship.