Many HIV-infected marginally housed individuals have difficulty engaging in health care. To investigate HIV health-related behaviour, 14 in-depth interviews with marginally housed HIV-infected individuals were conducted and analysed utilizing standard qualitative methodologies. The analysis was based on the Illness Representation Model, which describes five conceptual dimensions of illness: identification; cause; timeline; management; and consequences. A theoretical model of illness representation at two distinct time points emerged and included the two categories: 'didn't suspect and didn't believe it' and 'knew but needed proof'. In this study illness representation categories were found to evolve and change over time, and were associated with engagement in HIV care. This study may help guide programmes that focus on enhancing health-promoting behaviour and improving engagement in health care among marginally housed individuals.