Background Symptoms of anxiety and depression are common in adolescents with a loved one in an intensive care unit (ICU) and are known precursors of posttraumatic stress disorder (PTSD). Objectives To assess the prevalence of PTSD and associated factors in adolescent (age 12–17 years) relatives of patients in an ICU with an open visitation policy. Methods One year after the patient was discharged from the ICU, eligible adolescent relatives completed a satisfaction survey, anxiety history questionnaire, and psychometric evaluations (Hospital Anxiety and Depression Scale and 8-item Children’s Revised Impact of Event Scale). Results Thirty-two patients intubated for >2 days and with a Simplified Acute Physiology Score II >30 were included. Forty-six adolescents with first- to third-degree relationships to the patient, and in regular contact (≥monthly) with the patient before hospitalization, were enrolled. The prevalence of PTSD among the adolescents was 33%. Adolescents who visited the ICU were less likely to report feelings of regret than those who did not visit the ICU (2% vs 9%, P = .01). A past sense of threat (odds ratio [95% CI], 19.4 [1.9-201.2]; P = .01) and anxiety and depression symptoms (odds ratio [95% CI], 9.6 [1.4-63.7]; P = .02) were independent factors associated with probable PTSD. Conclusions A cautiously prepared open visiting policy should be maintained for adolescents with a relative in the ICU, because it could prevent feelings of regret and subsequent PTSD. Adolescents with risk factors should be screened and followed up.