This study reports the immediate psychological effects of a maritime explosion on 36 survivors, and the long term effects on 34 who were seen 3½ to 4½ years later. All were experienced seamen. The circumstamces of the accident were virtually identical for all. None received any systematic interim psychotherapy. All were guaranteed realistic compensation. Immediate effects in the majority of cases were appropriate to the circumstances, but long term investigation indicated appreciable deterioration in 71%, most frequent and most marked in the age group 36 and over. The long term psychological pictures were strikingly similar for all subjects.It was formulated from the data that repeated exposure to a potential danger apparently deprives the organism of its capacity to defend itself; when the danger has been converted to an actuality, it subsequently is perceived as a constant peril which is not amenable to repression, and the resultant reverberating anxiety accounts for the severity of the long term reactions.The data also supported the authors' previously stated hypotheses that the nature of the accident itself is a more significant determinant of post-traumatic psychological illness than the pre-accident personality; that such illness, if untreated, tends to worsen with time; and that litigation worries are a minor factor in producing the illness.It is suggested that the post-traumatic psychological states be considered diagnostic categories in themselves. This approach, because it shifts emphasis from the pre-accident personality to the accident itself, and recognizes that post-traumatic psychological damage is a discrete illness representing the failure of the individual's defense mechanisms, will best serve understanding and treatment of that illness.