Objectives: This study investigated the frequency of traumatic experiences, prevalence rates of ICD-11 post-traumatic stress disorder (PTSD) and complex PTSD (CPTSD), and overlap with ICD-10 classified disorders in outpatient psychiatry. Method: Overall, 165 Danish psychiatric outpatients answered the International Trauma Questionnaire, the Life Event Checklist, and the World Health Organization Well-being Index. ICD-10 diagnoses were extracted from the hospital record. Chi-square analysis, t-tests, and conditional probability analysis were used for statistical analysis. Results: Nearly, all patients (94%) had experienced at least one traumatic event. CPTSD (36%) was more common than PTSD (8%) and had considerable overlap with ICD-10 affective, anxiety, PTSD, personality, adjustment and stress-reaction disorders, and behavioural and emotional disorders with onset usually occurring in childhood and adolescence. ICD-11 PTSD overlapped with ICD-10 anxiety, PTSD, adjustment and stress-reaction disorders, and behavioural and emotional disorders with onset usually occurring in childhood and adolescence. A subgroup of patients with ICD-10 PTSD (23%) did not meet criteria for ICD-11 PTSD or CPTSD. Conclusion: Traumatic experiences are common. ICD-11 CPTSD is a highly prevalent disorder in psychiatric outpatients. One quarter with ICD-10 PTSD did not meet criteria for either ICD-11 PTSD or CPTSD. PTSD and CPTSD had considerable overlap with ICD-10 disorders.