Smoking harms health, particularly that of childhood cancer survivors, who face risk of pulmonary and cardiovascular diseases because of chemotherapy and radiotherapy to the chest. This nationwide study assessed smoking habits and reasons for smoking in adolescent survivors and healthy peers. As part of the Swiss Childhood Cancer Survivor Study, we sent a questionnaire to all Swiss resident survivors, who were aged 16-19 years. We compared smoking status and reasons for smoking between 511 survivors, 141 of their siblings, and 1,727 adolescents in a representative population-based study, the Tobacco Monitoring Switzerland (TMS). Current smoking was less prevalent in survivors (17%) and their siblings (17%) compared with TMS (32%). Survivors and TMS adolescents gave similar reasons for smoking. Stress control, smoking being a habit, and good taste were the reasons for smoking cited most often in both groups. Peer smoking was more important in survivors (49%) than in TMS (34%, P=0.004). Most important reasons for not smoking in both groups were smoking being unhealthy and not wanting to be addicted. In Switzerland, survivors smoke as often as their siblings but less than the general population. Peer smoking was a more important reason for smoking in survivors than in the general population, suggesting that reducing smoking in peers could result in a reduction of smoking in survivors. Overall, reasons for smoking were very similar, thus interventions to reduce smoking in survivors could be the same as those used in the general population.