This study sought to investigate correlates of past homelessness, especially mental illness and substance abuse, using data from the National epidemiological survey on alcohol and related conditions (NESARC), a large nationally representative survey conducted in 2001-2002. Multivariate analyses showed that the factors most strongly related to past homelessness were diagnoses of behavioral health conditions which showed consistently stronger association than sociodemographic characteristics, measures of economic well being, or general health indicators. The results presented here confirm in a nationally representative sample a number of single site studies that have also demonstrated the over representation of both mental illness and substance abuse disorders in homeless populations.