Caregiving for allogeneic hematopoietic stem cell transplant (Allo-HSCT) patients can be significantly burdensome. Caregiver well-being often mirrors patients' suffering. However, to our knowledge, this dyadic relationship has not been linked to patient outcome. Caregiver's objective and subjective sleep and overall distress before transplantation were hypothesized to be related to patient's time to engraftment in secondary analyses. Dyads (N = 124) were Allo-HSCT patients (mean [SD] age, 49.2 [12.7] years) and their caregivers (mean [SD] age, 52.7 [12.3] years). Caregiver's subjective sleep quality was measured via the Pittsburgh Sleep Quality Index, objective sleep was measured by actigraphy, and distress was measured by combining validated psychological measures. Both caregiver reports of worse sleep (β = .22; P < .05) and objective measurement of caregiver sleep patterns (higher sleep efficiency; less time awake after sleep onset) collected before engraftment significantly predicted shorter time to patient engraftment (β values = -.34 and .29, respectively; P values < .05). Caregiver distress was unrelated to engraftment (β = .14; P = .22). Despite limitations in available patient data, these findings appear to link caregiver well-being to patient outcome. This underscores the interrelatedness of the patient-caregiver dyad in Allo-HSCT. Future research should examine psychological and biomedical mediators. Given that caregiver well-being during the peritransplantation period was associated with patient outcome in this study, such findings highlight the need to address caregiver and patient well-being during Allo-HSCT. There may be potential to improve patient outcome by focusing on the caregiver, which nursing staff is well positioned to monitor.