In spite of extensive research on cancer and the family, little is known about the process of taking on the caregiving role when a family member is diagnosed with cancer. Family caregiver role acquisition was investigated in a grounded theory study of 19 family caregivers and 20 persons with cancer, interviewed across the course of chemotherapy. The initial analysis revealed that caregiving could not be understood in isolation from self-care in this sample and a reconceptualization of the target phenomenon was needed. Accordingly, the concept of family-based illness care was developed to encompass both self-care and caregiving. Twelve dimensions of family-based illness care and three prototypical patterns of illness care involvement were identified. Patterns of involvement fluctuated over time, a phenomenon labeled shifting patterns of self-care and caregiving. Shifting patterns of self-care and caregiving was so characteristic of illness care during chemotherapy that it became the core concept of the grounded theory. This conceptualization emphasizes the dyadic nature of family-based illness care during chemotherapy.