Objective Children undergoing cardiac surgery often experience traumatic situations related to their care and maybe at risk for developing posttramnatic stress disorder (PTSD). The purpose of this study was to examine children's responses to cardiac surgery and the factors that mediate responses. Study design Forty-three 5- to 12-year-old children undergoing cardiac surgery were evaluated pre- and postoperatively for PTSD. The effect of baseline cognitive level, temperament, family support, and length of intensive care unit (ICU) stay on PTSD symptomatology was examined. Results No child had PTSD pre-operatively. For the entire group, the number of PTSD symptoms increased significantly from pre- to postoperatively (Wilcoxon signed rank test; Z = -2.62, P < .001). PTSD symptoms increased in 10 (23%) children, decreased in I child, and did not change in 32 (74%) children. Five of the 43 children (12%) met criteria for PTSD postoperatively. Length of ICU stay was the only predictor of PTSD, P < .001. The number of PTSD symptoms increased if ICU stay exceeded 48 hours. Conclusions Children undergoing cardiac surgery are at risk for developing PTSD, especially if the ICU stay is prolonged. These findings provide a basis to develop risk stratification methods and targeted interventions for this population.