Objective: The aim of this investigation was to test whether interdependence in dyads living with breast cancer could account for person-partner crossover effects in distress outcomes. Method: The sample consisted of 95 dyads with early-stage breast cancer. By using reciprocal dyadic data from women with breast cancer and their partners, we fit a structural equation model of the actor-partner interdependence model to examine the interaction of participants' depression and stress in predicting their partner's health outcomes. Results: Results revealed a pattern of influence whereby the interaction of high levels of depression coupled with high levels of stress in women with breast cancer was associated with lowered physical health and well-being in their partners. Although depression seemed to be the key mechanism in predicting distressing outcomes, when depression was combined with any additional stress, the level of physical distress was significantly greater. Results provided preliminary empirical support for crossover effects in the physical well-being of close relational partners in a cancer-related context. Further, results showed that distressing outcomes need not be limited to emotional distress but can also include physical distress. Conclusions: The findings from this study illustrate why it is not sufficient to concentrate care solely on the patient with cancer. Monitoring the social well-being of patients as they go through the cancer process could be as important as assessing their psychological state or other peripheral biomarkers. This line of inquiry would be advanced by including methods other than self-report in assessments of psychological and physical health. (Contains 1 figure and 1 table.)