Idiopathic environmental intolerance (IEI), also known as multiple chemical sensitivity (MCS), is defined as a chronic polysymptomatic condition that cannot be explained by an organic disease. Previous studies suggest that IEI may be a variant of somatoform disorders (SFD), because both disorders overlap with respect to symptoms and psychological features of somatization. However, little is known about the short- and medium-term outcome of IEI and psychological outcome predictors. Two clinical groups (IEI and SFD) and a comparison group (CG) were followed through 32 mo to assess both the outcome, and the extent to which trait anxiety and somatic symptom attribution (assessed at first examination) predict outcome presented 12 and 32 mo later. Outcome measures were the number of self-reported IEI symptoms, IEI triggers, IEI-associated functional impairments, and the number of somatoform symptoms. In addition, the course of the 2 syndromes over the 32-mo follow-up period was investigated with standardized screening scales. The 3 diagnostic groups consisted of 46 subjects with IEI, 38 subjects with SFD but without IEI, and 46 subjects (CG) with neither IEI nor SFD. Syndrome stability was high over the 32-mo follow-up period, and at both follow-ups IEI and non-IEI subjects differed on all IEI outcome measures (symptoms, triggers, functional impairments). Both trait anxiety and somatic attribution (the tendency to attribute common somatic complaints to an illness) predicted outcome. In addition, somatic attribution was found to partially mediate the effect of trait anxiety on outcome in the IEI group. In conclusion, these results suggest that IEI is a chronic and disabling condition and that trait anxiety contributes to the maintenance of the disorder via somatic attributions.