The purpose of this study is to examine the associations between parent- and child-reported illness uncertainty and psychosocial functioning among solid organ transplant recipients. Youths who received liver or kidney transplantations and their caregivers completed measures of illness uncertainty, youth depressive symptoms, youth anxiety, and youth psychosocial functioning. Regression analyses indicated that youth-reported uncertainty was associated with both youth-reported depressive symptoms and anxiety and with parent-reported adaptive functioning and behavioral functioning. Parent-reported uncertainty was associated with parent-reported overall behavioral functioning, adaptive functioning, and internalizing problems. Results extend previous findings regarding illness uncertainty, and suggest that educational interventions to reduce uncertainty might be of benefit to families.