Previous studies have shown that children and adolescents exposed to traumatic experience in a disaster can suffer from high levels of post-traumatic stress. The present paper is the first a series reporting on the long-term follow-up of a group of young adults who as teenagers had survived a shipping disaster—the sinking of the “Jupiter” in Greek waters—between 5 and 8 years previously. The general methodology of the follow-up study as a whole is described, and the incidence and long-term course of Post-Traumatic Stress Disorder (PTSD). It is the first study of its kind on a relatively large, representative sample of survivors, using a standardised diagnostic interview, and comparing survivors with a community control group. Survivors of the Jupiter disaster (N = 217), and 87 young people as controls, were interviewed using the Clinician Administered PTSD Scale (CAPS). Of the 217 survivors, 111 (51.7%) had developed PTSD at some time during the follow-up period, compared with an incidence in the control group of 3.4% (N = 87). In the large majority of cases of PTSD in the survivors for whom time of onset was recorded, 90% (N = 110), onset was not delayed, being within 6 months of the disaster. About a third of those survivors who developed PTSD (30%, N = 111) recovered within a year of onset, through another third (34%, N = 111) were still suffering from the disorder at the time of follow-up, between 5 and 8 years after the disaster. Issues relating to the generalisability of these findings are discussed.