Background: The impact of pediatric hematopoietic stem cell transplant (HCT) on family functioning varies, but little is known about how the timing of HCT in children's treatment course contributes to this variability. This study examines how preexisting child, sibling, and family problems, the length of time between diagnosis to HCT, and children's age at HCT are associated with family and caregiver functioning. Procedure: Caregivers (n = 140) of children (≤18 years old) scheduled to undergo their first HCT completed the Psychological Assessment Tool-HCT and the Impact on Family Scale. Treatment information was extracted from electronic medical records. A bootstrapped multivariate path analysis was used to test the hypotheses. Results: More preexisting family problems related to greater caregiver perceived negative impact of their child's HCT across family and caregiver functioning domains. Less time between diagnosis and HCT was associated with greater caregiver personal strain, particularly for those with younger children undergoing HCT. Younger child age at HCT was also associated with a larger negative impact on family social functioning. Conclusions: Families with preexisting problems are the most at-risk for experiencing negative impacts related to their child's HCT. The timing of a child's HCT within their treatment course and the child's age during HCT may impact families' social functioning and caregiver adjustment. Screening families for preexisting family problems, particularly for families with young children or who are abruptly learning of their child's need for an HCT, may assist providers in identifying families who would benefit from earlier or more intensive psychosocial support.