A clearer understanding of the association between a biomarker of long-term stress reactivity and family functioning among pediatric cancer survivors may guide both survivorship research and clinical practice. The current study examined the relationship between a long-term measure of hypothalamic–pituitary–adrenal (HPA) activity (cortisol concentration; CORTHAIR) and parent-reported family functioning (Family Environment Scale; FES) in a cross-sectional sample of survivors (n = 26) and controls (n = 53). Child CORTHAIR was not different in survivors and controls, though treatment severity was significantly related to child survivor CORTHAIR. Child CORTHAIR and parent CORTHAIR were positively correlated. Cancer survivor parents reported greater FES Organization. Child CORTHAIR was inversely associated with FES Independence, while parent CORTHAIR was inversely correlated with FES Organization. Parent CORTHAIR and FES Independence were significant and unique predictors of child CORTHAIR. Our results provide preliminary evidence for a relationship between a stress biomarker, child CORTHAIR, and family functioning among pediatric cancer survivors and controls. [ABSTRACT FROM AUTHOR]