This article examines the multifaceted interactions among homelessness, HIV, substance abuse, and gender. Data were collected on 1,366 chronic drug users using a nationally standardized validated instrument within the Miami CARES project of a multisite federally funded program. HIV testing accompanied by pretest and posttest counseling was conducted on-site by certified phlebotomists and counselors. In addition to descriptive analyses and corresponding tests of significance, logistic regression analyses were used to clarify the complex associations between the outcome variables of homelessness and HIV, recognizing difficulties of determining temporal sequence. HIV infection was found to be 2.35 times more prevalent among homeless women than homeless men and significantly higher for homeless women. The findings indicate that among women, homelessness and HIV have a highly interactive effect increasing the vulnerability of this population and thus rendering them an extremely important priority population on which to focus public health efforts and programs.