Our objectives were to explore the relation between food insufficiency and the health of clientele who frequent services for the homeless; and to compare that clientele with the general population in Quebec and in Canada on the association between food insufficiency and health. "Ibis study is a secondary data analysis of the 1998-1999 Quebec Survey of the clientele of services for homeless people conducted in Montreal and Quebec City. A representative sample of the daily adult population frequenting shelters, soup kitchens and drop-in centres for homeless people-in these cities had been selected according to a complex survey design between December 1998 and August 1999. 458 respondents were interviewed. The questionnaire, administrated in a face-to-face interview, comprised over one hundred standardized questions covering many aspects of the respondents' demographic, social, cultural and economics, and lifestyle behaviour as well as their mental and physical health status. For the comparison with the general population in Quebec and in Canada, data file of the cycle 2 1996-1997 National Population Health Survey was used. This particular survey was chosen because it included a similar question on food insufficiency and it was reasonably close in time. The results of this study highlight that food insufficiency is statistically associated with poorer mental and physical health in currently or formerly homeless persons. The effects of food insufficiency on their health would be even greater than that observed in the general population. They also highlight that housing matters for food sufficiency. These results provide policy makers with a better understanding of the relationship between homelessness, food insufficiency and health so as to improve policies and programs directed to homeless persons. The potential role of food security in promoting better physical, mental and social health among this population should be urgently examined.