This study was conducted to examine mood state and quality of life (QOL) in patients who had undergone a mastectomy. The study used a descriptive design and was conducted in outpatient oncology clinics in northeastern Iran. Using the Profile of Mood States and the Ferrans and Powers Quality of Life Index, the mood state and QOL of patients who had undergone a mastectomy were examined. The results indicate that participants had low mood state and QOL; in addition, a negative correlation was found between the two. The mood state was a predictor of participants' QOL. A screening system for patients who have undergone a mastectomy may help nurses evaluate patients' mood states, identify the symptoms of mood disturbances, and refer patients to the appropriate support services.