Acute stress disorder (ASD) refers to the symptoms associated with posttraumatic stress disorder (PTSD) within the first four weeks following the traumatic event. Recent theoretical models suggest that early detection of ASD provides an opportunity to implement early interventions to prevent the development of PTSD or ameliorate its symptomatology. The aim of the present study was the evaluation of the efficacy of an ASD treatment for earthquake victims, which would serve as an early intervention for PTSD. A single-case (n = 1) quasi-experimental design was used, with pre and post-assessments, as well as one, three and six-month follow-ups, with direct treatment replications. Fourteen participants completed the treatment and the follow-up measurements. The results obtained using a single-case analysis showed significant clinical improvement and clinically significant change when employing a clinical significance analysis and the reliable index of change. Statistical analyses of the dataset displayed statistically significant differences between the pre and post-assessments and the follow-up measures, as well as large effect sizes in all clinical measures. These results suggest that the treatment was an efficacious early intervention for PTSD during the months following the traumatic event, although some relevant study limitations are discussed in the text.