The objective of this study was to describe health services utilization by homeless and housed poor adults stratified by six-month primary sheltering arrangements. The primary method used in this study was a cross-sectional survey of 373 homeless adults. Interviews at twenty-four community-based sites (in Allegheny County, Pennsylvania) assessed demographic and clinical characteristics, reasons for homelessness, functional status and social support networks, and health services utilization during the previous six months. Multivariate logistic regression analysis identified factors independently associated with health services utilization. Subjects were classified as unsheltered, emergency-sheltered, bridge-housed, doubled-up, and housed-poor. The median age of the subjects was 38.4 years; 78.6 percent were African American and 69.9 percent had health insurance. Overall, 62.7 percent reported health services use in the past six months, with significantly more use among emergency-sheltered and bridge-housed subjects than among unsheltered subjects. The study concludes that health services use among the homeless is substantial and is independently associated with sheltering arrangement, comorbid illness, race, health insurance, and social support.