Background: Patient encounters for medically unexplained physical symptoms are common in primary health care. Somatization ('experiencing and reporting unexplained somatic symptoms') may indicate concurrent or future disability but this may also partly be caused by psychiatric disorders. The aim of this study was to examine the cross-sectional and longitudinal association between somatization and disability in primary care patients with and without anxiety or depressive disorder. Methods: Data were obtained from 1545 primary care patients, participating in the longitudinal Netherlands Study of Depression and Anxiety (NESDA). Somatization was assessed using the somatization scale of the Four-Dimensional Symptom Questionnaire (4DSQ). Disability was determined by the WHO Disability Assessment Schedule 2.0 (WHO-DAS II). The relationships between somatization and both the total and subdomain scores of the WHO-DAS II were measured cross-sectionally and longitudinally after one year of follow-up using linear regression analysis. We examined whether anxiety or depressive disorder exerted a modifying effect on the somatization-disability association. Results: Cross-sectionally and longitudinally, somatization was significantly associated with disability. Somatization accounted cross-sectionally for 41.8% of the variance in WHO-DAS disability and, longitudinally, for 31.7% of the variance in disability after one year of follow-up. The unique contribution of somatization to disability decreased to 16.7% cross-sectionally and 15.7% longitudinally, when anxiety and/or depressive disorder was added to the model. Conclusion: Somatization contributes to the presence of disability in primary care patients, even when the effects of baseline demographic and health characteristics and anxiety or depressive disorder are taken into account.