ObjectiveIt is not clear why psychiatric disorders are more prevalent in the functional somatic syndromes than other general medical illnesses. This study assessed the correlates of psychiatric disorders in 3 functional syndromes and 3 general medical illnesses in a population-based sample.MethodsThe Lifelines cohort study included 122,366 adults with relevant data for 6 self-reported conditions: irritable bowel syndrome (IBS), fibromyalgia, chronic fatigue syndrome (CFS), inflammatory bowel disease (IBD), rheumatoid arthritis (RA), and diabetes. For each condition the proportion with a DSM-IV psychiatric disorder was assessed. In a cross-sectional design, logistic regression identified at baseline the variables most closely associated with current psychiatric disorder in participants with a pre-existing medical or functional condition. In a separate analysis the prevalence of psychiatric disorder prior to onset of these conditions was assessed. This was a longitudinal study with psychiatric disorder assessed at baseline in participants who subsequently developed a general medical or functional condition between baseline and follow-up.ResultsThe prevalence of psychiatric disorder was higher (17–27%) in the functional somatic syndromes than the general medical illnesses (10.4–11.7%). The variables closely associated with psychiatric disorder were similar in the functional syndromes and general medical illnesses: stressful life events, chronic personal health difficulties, neuroticism, poor perception of general health, impairment of function due to physical illness and reported previous (lifetime) psychiatric disorder. The prevalence of psychiatric disorder prior to development of these disorder was similar to that of established disorders.ConclusionDespite the difference in prevalence, the correlates of psychiatric disorders were similar in functional and general medical disorders and included predisposing and environmental factors. The increased rate of psychiatric disorder in functional somatic syndromes appears to be evident before onset of the syndrome.