OBJECTIVE The objective of this study was to examine the relationship between combat-related posttraumatic stress disorder (PTSD) and comorbid DSM-III-R psychiatric diagnoses to determine commonalities in risk factors, relative onsets and the role of combat exposure. METHODS An epidemiological cohort study using standardised psychiatric, social and health interviews was undertaken with a national random sample of male Australian Army Vietnam veterans. Interviews and searches of military records yielded risk factors for PTSD, which were examined for association with each psychiatric diagnosis. Relative onsets of PTSD and each Diagnostic Interview Schedule diagnosis were compared. Comorbidity odds ratios were adjusted for combat exposure effects using logistic regression, and the relation between each diagnosis and combat was assessed after controlling for PTSD. RESULTS Commonality of risk factor profile was evident for several diagnoses, and for many their onset preceded PTSD onset. Combat was independently related to only a few diagnoses after controlling for PTSD, and PTSD remained strongly associated with several conditions after controlling for combat exposure. CONCLUSIONS The analysis suggests that the disorders that may constitute risk factors or vulnerabilities for PTSD comprise depression and dysthymia, antisocial personality disorder, agoraphobia and simple phobia, while those that may be consequent on PTSD are panic and generalised anxiety disorder, drug use disorders and somatoform pain disorder. Alcohol and drug use disorders and social phobia may have a mixed aetiology, while obsessive-compulsive disorder may be serendipitously related to PTSD through an association with risk of combat. Gambling disorder is unrelated.