['There is growing international concern that food insecurity may negatively impact antiretroviral (ARV) treatment outcomes, but no studies have directly evaluated the effect of food insecurity on viral load suppression and antiretroviral adherence. We hypothesized that food insecurity would be associated with poor virologic response among homeless and marginally housed HIV-positive ARV-treated patients.', 'This is a cross-sectional study.', 'Participants were ARV-treated homeless and marginally housed persons receiving adherence monitoring with unannounced pill counts in the Research on Access to Care in the Homeless (REACH) Cohort.', 'Food insecurity was measured by the Household Food Insecurity Access Scale (HFIAS). The primary outcome was suppression of HIV viral RNA to <50 copies/ml. We used multivariate logistic regression to assess whether food insecurity was associated with viral suppression.', 'Among 104 participants, 51% were food secure, 24% were mildly or moderately food insecure and 25% were severely food insecure. Severely food insecure participants were less likely to have adherence > or =80%. In adjusted analyses, severe food insecurity was associated with a 77% lower odds of viral suppression (95% CI = 0.06-0.82) when controlling for all covariates. In analyses stratified by adherence level, severe food insecurity was associated with an 85% lower odds of viral suppression (95% CI = 0.02-0.99) among those with < or =80% adherence and a 66% lower odds among those with >80% adherence (95% CI = 0.06-1.81).'] Is food insecurity associated with incomplete HIV RNA suppression among homeless and marginally housed HIV-infected individuals in San Francisco?