BACKGROUND: The Hospital Anxiety and Depression Scale (HADS) is a widely used instrument to measure psychological morbidity in cancer patients. This study aimed to translate and test the reliability and validity of the Iranian version of the HADS. METHODS: The English language version of the HADS was translated into Persian (Iranian language) and was used in this study. The questionnaire was administered to a consecutive sample of 167 breast cancer patients and statistical analysis was performed to test the reliability and validity of the HADS. RESULTS: In general the Iranian version of the HADS was found to be acceptable to almost all patients (99%). Cronbach's alpha coefficient (to test reliability) has been found to be 0.78 for the HADS anxiety sub-scale and 0.86 for the HADS depression sub-scale. Validity as performed using known groups comparison analysis showed satisfactory results. Both anxiety and depression sub-scales discriminated well between sub-groups of patients differing in clinical status as defined by their disease stage. CONCLUSION: This preliminary validation study of the Iranian version of the HADS proved that it is an acceptable, a reliable and valid measure of psychological distress among cancer patients.