Purpose:To assess adolescent and young adult siblings' perception of social support prior to and following the loss of their brother or sister to cancer, 2 to 9 years earlier, and their anxiety at follow-up. Method: In 2009, 174 (73%) bereaved siblings (12-25 years) participated in a nationwide, long-term follow-up study in Sweden using an anonymous study-specific questionnaire. The Hospital Anxiety and Depression Scale was used to measure self-assessed anxiety. Results: Siblings had a higher risk of anxiety if they perceived their need for social support was unsatisfied during their brother or sisters' last month before death, relative risk (RR) = 3.6 (95% confidence interval [CI] = 1.8-7.3); time after death, RR = 2.9 (95% CI = 1.5-5.6); and at follow-up, RR = 3.8 (95% CI = 2.0-7.2). Furthermore, a higher risk for anxiety was shown for siblings if they did not perceive that their parents and neighbors cared for them after their brother or sisters' death, RR = 2.7 (95% CI = 1.3-5.5), RR = 5.4 (95% CI = 1.3-21.9), respectively. Conclusion: Bereaved siblings had a greater probability to report self-assessed anxiety if they perceived that their need for social support was not satisfied prior to and following death. Information from both nurses and other health care professionals to families about the impact of social support may contribute to lessen the siblings' risk of anxiety.