The main aim of this study was to investigate the occurrence of cancer-related posttraumatic stress disorder (PTSD) among parents of children on cancer treatment. A longitudinal design with assessments at one week (T1), two (T2), and four (T3) months after the child’s diagnosis was used. Two hundred and fourteen parents (107 mothers, 107 fathers) participated at T1-T3. The PTSD Checklist Civilian (PCL-C), a self-report screening instrument for PTSD, was answered by parents over the telephone. According to the PCL-C symptom criteria method 33%, more mothers than fathers, score as potential cases of acute stress disorder (ASD) at T1, whereas 28% as potential cases of PTSD at T2 and 22% at T3. The levels of acute- and posttraumatic stress symptoms show a linear, descending pattern, and mothers report higher levels than fathers. Half of the parents who score as potential cases of ASD a week after the child’s diagnosis score as potential cases of PTSD four months later. The findings illustrate that a group of parents of children with cancer experience serious psychological distress related to their child’s disease. A traumatic stress perspective on childhood cancer should be applied to paediatric oncology care and appropriate psychosocial interventions should be offered to parents when needed.