Acute and subacute post-traumatic stress reactions are reported among 246 employees of an industrial factory which was severely damaged by an explosion and fire. Sixty-six A-subjects had narrow escape experiences (high stress exposure group), while 59 B-subjects were less severely exposed (medium stress exposure group). The 121 C-subjects were not present at work when the explosion occurred (low stress exposure group). A response rate of 97.6% was achieved at the primary examination, and a 100% response at the 7 months follow-up. The frequency and intensity of post-traumatic stress reactions were linked to the severity (A,B,C) of the stress exposure; specific post-traumatic anxiety reactions reported by more than 80% of A-subjects. The reactions appeared immediately or within hours, only 5% of A had delays of a few weeks. While 24.3% of A had State Anxiety Inventory scores 1 week post-disaster higher than 60, 8.5% of B and 2.5% of C had similar scores. Depressive reactions, social withdrawal, guilt, shame and irritability were less frequent, and appeared nearly always concomitant with anxiety symptoms. While the anxiety symptoms made up a tight knit syndrome, the less frequent non-anxiety symptoms were linked to the post-traumatic anxiety syndrome. The subjects' fears reflected the trauma, they feared inanimate objects, and there were hardly any paranoid ideations. The disaster exposure of the A and B but not of the C group members constituted a stressor which fulfilled the PTSD stressor criterion of the DSM III R. A minority of the C group developed a post-traumatic stress syndrome. After 7 months, all 30 post-traumatic stress reactions were more frequent and severe in the A than B group which again differed from the C group. Irritability was the only post-traumatic stress reaction that increased in frequency and intensity during the 7 months observation period. The findings represent evidence that supports the face validity, descriptive and construct validity of the PTSD diagnosis.