AIMS: To investigate the extent that common psychological conditions contribute to lost work among individuals with musculoskeletal and ill defined conditions. METHODS: Cross sectional health and work related survey evaluating Gulf War veterans seeking Department of Defense health care for Gulf War related health concerns. Ordered probit models were used to study whether a provider diagnosed musculoskeletal condition (ICD-9 codes 710-739) or "signs, symptoms, and ill defined conditions" (ICD-9 codes 780-799) have an effect on recent lost work over the previous 90 days in the presence of one or more psychological conditions (ICD-9 codes 290-320) after controlling for sociodemographic variables. RESULTS: Bivariate analyses revealed that musculoskeletal conditions, ill defined conditions, and psychological conditions were positively associated with lost work. Multivariate analyses showed an independent effect of both psychological conditions and musculoskeletal conditions. A significant interaction existed between psychological conditions and musculoskeletal conditions: the presence of a coexisting psychological condition considerably increased the likelihood that a musculoskeletal disorder resulted in lost work, or vice versCONCLUSIONS: Psychological conditions appear to be an important contributor to absenteeism among individuals with musculoskeletal and ill defined conditions. A limitation of the cross sectional design was the inability to sequence the onset of conditions.