OBJECTIVES: To explore the psychologic impact of an ICU stay on relatives and to determine patient and relative factors, including their pre-ICU mental health status, associated with psychologic symptoms 3 months after ICU admission. DESIGN: A prospective, exploratory, longitudinal cohort study. SETTING: A 12-bed ICU in a Dutch tertiary teaching hospital. PATIENTS: The relatives of ICU patients admitted between March 2018 and December 2019. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Relatives completed the Hospital Anxiety and Depression Scale shortly after patients’ ICU admission referring to the week before ICU admission and 3 months later, together with the Impact of Event Scale-Revised, assessing posttraumatic stress. A total of 387 were eligible of which 78 (20%) responded. Almost a quarter reported an increase of anxiety (23.1%) and depression (24.4%) between baseline and 3 months follow-up. One in six (17.9%) reported posttraumatic stress. Anxiety, depression, and posttraumatic stress at 3 months follow-up were significantly associated with preexisting anxiety and depression. Patients’ factor associated with depression 3 months follow-up in relatives was length of ICU stay (odds ratio, 1.09; 95% CI, 1.02–1.16). CONCLUSIONS: Relatives with preexisting psychologic symptoms before ICU admission are at risk to develop anxiety, depression, or posttraumatic stress shortly after ICU discharge of the patient. Screening and identification in an early stage enables early interventions to prevent relatives from worsening mental health during and after ICU admission.