Background: Previous studies found notable rates of post-traumatic stress symptoms (PTSS) and post-traumatic stress disorder (PTSD) in pediatric patients and their parents and suggest a significant association between child and parent PTSS. However, little is known about mutual influences between child and parental PTSS over time. This study prospectively examined the presence of PTSS and PTSD and the mutual influence of child and parental PTSS in a large sample of pediatric patients with different medical conditions. Methods: A total of 287 children (aged 6.5-16 years) and their mothers (n = 239) and fathers (n = 221) were assessed at 5-6 weeks and 1 year after an accident or a new diagnosis of cancer or diabetes mellitus type 1 in the child. Results: At the first assessment 11.1% and at the second assessment 10.2% of the children had moderate to severe PTSS. At 5-6 weeks 29.3% of mothers and 18.6% of fathers met criteria for PTSD. At 1 year the rates were 14.6% for mothers and 7.9% for fathers. There were considerable differences of PTSS among different medical diagnostic groups in children and parents. Mothers were more vulnerable than fathers. Structural equation analysis revealed that initially high PTSS in mothers and fathers were longitudinally related to poorer recovery from PTSS in the child. Cross-lagged effects from the child to the parents and from one parent to the other were not significant. Conclusions: This study highlights the long-term influence of parental PTSS on the child's recovery after trauma and calls for a family systems approach and for early interventions in the treatment of traumatized pediatric patients.