The main aim of this study was to investigate factors associated with a delayed-onset posttraumatic stress disorder (PTSD) after the intensive care unit (ICU) treatment of patients with a chronic critical illness (CCI). Patients (n = 97) with critical illness polyneuropathy or critical illness myopathy were interviewed via the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, 4th Edition. The diagnosis of the acute stress disorder was assessed within 1 month (t1), the diagnosis of PTSD at 3 (t2) and 6 (t3) months after transfer from the acute care ICU to the post-acute ICU. Patients showing a delayed-onset or persistent course of PTSD were subsumed in one group; 24.7% (n = 24) showed a delayed-onset PTSD. Significant risk factors were as follows: the severity of the medical illness, the perceived fear of dying at the ICU, the number of traumatic memories from the ICU, and the presence of a coronary heart disease. Every fourth patient with CCI showed a delayed-onset PTSD up to 6 months after the ICU treatment. Markers for a delayed-onset PTSD should already be assessed at the time of discharge from the ICU.