In this study, the authors investigated the relationship between acute stress disorder (ASD) and posttraumatic stress disorder (PTSD) following cancer diagnosis. Patients who were recently diagnosed with 1st onset head and neck or lung malignancy (N = 82) were assessed for ASD within the initial month following their diagnosis and reassessed (n = 63) for PTSD 6 months following their cancer diagnosis. At the initial assessment, 28% of patients had ASD, and 32% displayed subsyndromal ASD. At follow-up, PTSD was diagnosed in 53% of patients who had been diagnosed with ASD and in 11% of those who had not met criteria for ASD; 36% of patients with PTSD did not initially display ASD. In this study, the authors question the use of the ASD diagnosis to identify recently diagnosed patients at risk of PTSD.