International studies indicate high prevalence rates of post-traumatic stress disorder within homeless populations. In Australia, studies indicate high rates of trauma among homeless adults, yet post-traumatic stress disorder has not been investigated in homeless Australian adults. The primary aim of this project was to determine the prevalence of post-traumatic stress disorder among homeless adults in Sydney. Further, another aim of the study was to determine whether the onset of post-traumatic stress disorder preceded the first episode of homelessness or was a consequence of homelessness.