The diagnostic criteria of delayed-onset PTSD is that the onset of PTSD symptoms should occur at least 6 months after the traumatic event (DSM-IV; American Psychiatric Association, 2000). In a meta analysis across 11 countries, 25% of all PTSD cases were classified as delayed-onset PTSD (Smid et al., 2009). Existing research on delayed-onset PTSD has predominantly used US data, with a dearth of prospective research in UK military samples.

This study investigated the prevalence of delayed-onset PTSD in UK military personnel and examined the socio-demographic, military and psychological characteristics of delayed-onset PTSD.

The sample comprised 1397 participants from a two phase prospective cohort study of UK military personnel. Delayed-onset PTSD was categorised as participants who did not meet the criteria for probable PTSD (assessed by the PCL-C) at phase 1, but who met the criteria by phase 2.

3.5% of participants met the criteria for delayed-onset PTSD. Subthreshold PTSD, common mental disorder (CMD), poor/fair self-reported health and multiple physical symptoms at phase 1, and the onset of alcohol misuse or CMD between phases 1 and 2, were associated with delayed-onset PTSD.

Delayed-onset PTSD exists in this UK military sample. Military personnel who developed delayed-onset PTSD were more likely to have psychological ill-health at an earlier assessment and clinicians should be aware of potential co-morbidity in these individuals, including alcohol misuse. Leaving the military, or experiencing relationship breakdown, was not associated.