Purpose. The purpose of this study was to identify direct, mediating, and moderating relationships of mood disturbance, symptom experience, and attentional function in Korean women with breast cancer based upon a middle-range theory of unpleasant symptoms. Methods. This study used a cross-sectional, correlational design. A convenience sample of 125 women receiving chemotherapy for breast cancer was recruited from a university hospital in South Korea. The women completed questionnaires on mood disturbance, symptom experience, and attentional function using the Linear Analogue Self-Assessment Scale, the Symptom Experience Scale, and the Attentional Function Index, respectively. Results. Each mood disturbance and symptom experience showed a significant relationship with attentional function. Symptom experience did not act as a mediator between mood disturbance and attentional function, but it did act as a moderator: patients with a higher level of mood disturbance exhibited a lower level of attentional function when their symptoms were at the level of medium, but not when their symptoms were either high or low. Conclusion. This suggests that clinical interventions for attenuating the influence of mood disturbance on attentional function may be effective only in women experiencing medium level of symptoms.