Objectives Post‐traumatic stress symptoms (PTSS) have been identified as a meaningful indicator of distress in cancer survivors. Distinct from young adult survivors of childhood cancer, young people diagnosed with cancer as adolescents and young adults (AYAs) face unique psychosocial issues; however, there is little published research of PTSS in the AYA population. This study examines prevalence and predictors of PTSS among AYAs with cancer. Methods As part of a longitudinal study of AYAs with cancer, 151 patients aged 15–39 years completed mailed surveys at 6 and 12 months post‐diagnosis. Severity of PTSS was estimated at 6 and 12 months post‐diagnosis. Multiple regression analyses were conducted to investigate the predictive effects of socio‐demographic and clinical characteristics on changes in PTSS over time. Results At 6 and 12 months, respectively, 39% and 44% of participants reported moderate to severe levels of PTSS; 29% had PTSS levels suggestive of post‐traumatic stress disorder. No significant differences in severity of PTSS between 6 and 12 months were observed. Regression analyses suggested that a greater number of side effects were associated with higher levels of PTSS at 6 months. Currently receiving treatment, having surgical treatment, diagnosis of a cancer type with a 90–100% survival rate, remaining unemployed/not in school, and greater PTSS at 6 months were associated with higher levels of PTSS at 12 months. Conclusions Post‐traumatic stress symptoms were observed as early as 6 months following diagnosis and remained stable at 12‐month follow‐up. The development of early interventions for reducing distress among AYA patients in treatment is recommended. Copyright © 2012 John Wiley & Sons, Ltd.