Objective: To develop, evaluate, and apply a questionnaire-based instrument for investigation of specific symptoms in idiopathic environmental intolerance (IEI), called the Idiopathic Environmental Intolerance Symptom Inventory (IEISI).
Methods: Participants with IEI to chemicals responded to 82 candidate symptoms and to three subscales of the Quick Environmental Exposure and Sensitivity Inventory (QEESI) at a test (n = 207) and retest (n = 193) occasion.
Results: The 27 most commonly reported symptoms were selected and grouped into five symptom categories. Internal consistency, test–retest reliability, and concurrent validity were found to be satisfying. Cluster analysis identified two subgroups of IEI to chemicals.
Conclusions: The results provide support for the IEISI being a reliable, valid, and fast tool for the study of specific symptom prevalence in IEI and encourage further study of subgroups.