General systems theory and Bodenmann’s theory of dyadic coping (Bodenmann, 1997) provided the framework for exploring the impact of life-threatening illness on the dyadic relationship. The sample included 193 dyads experiencing the stress of treatment by bone-marrow transplantation (BMT) for cancer which had not responded to first-line therapies. A prospective design over 12 months included four measurements at designated periods in the treatment process. Except for the symptom checklist, which was completed only by the recipient, data were obtained on each measure from both partners at each time point. The primary outcome was dyadic adjustment measured by the Dyadic Adjustment Scale (DAS; Spanier, 1976). A general linear mixed model indicated that dyadic adjustment was stable over time for BMT recipients and family caregivers; however, caregivers had lower mean values on dyadic adjustment than recipients, and higher mean values on negative affect. Bayesian path analysis was used to test actor–partner interdependence models reflecting the theory of dyadic coping. Models exhibited adequate fit to the data and indicated that apart from baseline dyadic adjustment, partner-related coping had the greatest positive impact on dyadic adjustment over the trajectory for both recipients and their caregivers. Limitations in resources for health care occurring on a national level have increased the extent of in-home care for acutely ill family members, including BMT recipients. This has serious implications for the mental health of the caregiver and for the integrity of the family system. Mental health assessment and integration of supportive interventions would be important in the prevention of secondary psychosocial morbidity.