Objective: The experience of lifetime trauma among homeless women in the USA is well documented. Less information is available concerning homeless men. There are no prevalence studies concerning lifetime trauma among homeless people in Australia. Our aim was to assess the lifetime prevalence of trauma as reported by homeless men and women in Sydney. Method: We interviewed 119 men and 38 women who were visiting or residing at the seven largest refuges for homeless people in inner Sydney, using the lifetime trauma section of the Composite International Diagnostic Interview. Results: All women and over 90% of men reported at least one event of trauma in their life. Fifty-eight per cent suffered serious physical assault and 55% witnessed someone being badly injured or killed. Half the women and 10% of men reported that they had been raped. Conclusion: The experience of at least one lifetime event of trauma is almost universal among homeless people in Sydney and is considerably higher than for the USA general population. Reasons for such high prevalence rates are discussed. Depression and posttraumatic stress disorder are associated with a history of trauma. Health professionals need to be aware of past events of trauma among individuals who are homeless. (abstract from PUBMED)