Background: Families of cancer patients in the ICU often experience severe stress. Understanding their experience is important for providing family-centered care during this difficult period. Little is known about the experience of families of cancer patients admitted to the ICU. This study evaluated the prevalence of posttraumatic stress symptoms (PTSS) among families of cancer patients admitted to the ICU. Methods: We carried out a longitudinal study at a teaching and advanced treatment hospital. Participants were 23 family members of 23 ICU patients. Family members provided demographic data, electronic medical records of patients, and completed the Impact of Event Scale-Revised (IES-R), the Center for Epidemiologic Studies Depression Scale (CES-D), and the State-Trait Anxiety Inventory Form X (STAI-state, trait). Results: Mean total IES-R total score, IES-R re-experience score, IES-R avoidance score, and STAI-state score within 24 h of ICU admission and 3 months later differed significantly. The IES-R score of families of patients with recurrent cancer was significantly higher than the score of families of patients with an original cancer diagnosis (t = 2.63, p = 0.029). For two-way analysis of variance, time point was significantly associated with IES-R score (F = 1.751, p = 0.011, df = [1]). Conclusions: Families of recurrent cancer patients admitted to the ICU experience serious PTSS within 24 h of admission. It is important that appropriate psychiatric support be provided to family members of these patients.