Health services research consistently confirms the benefit of insurance coverage on the use of health services sought in the USA. However, few studies have simultaneously addressed the multitude of competing and unmet needs specifically among unstably housed persons. Moreover, few have accounted for the fact that hospitalization may lead to obtaining insurance coverage, rather than the other way around. This study used marginal structural models to determine the longitudinal impact of insurance coverage on the use of health services and antiretroviral therapy (ART) among HIV-positive unstably housed adults. The impact of insurance status on the use of health services and ART was adjusted for a broad range of confounders specific to this population. Among 330 HIV-positive study participants, both intermittent and continuous insurance coverage during the prior 3-12 months had strong and positive effects on the use of ambulatory care and ART, with stronger associations for continuous insurance coverage. Longer durations of continuous coverage were less robust in affecting emergency and inpatient care. Race and ethnicity had no significant influence on health services use in this low-income population when confounding due to competing needs was considered in adjusted analyses. Given that ambulatory care and ART are factors with substantial potential impact on the course of HIV disease, these data suggest that securing uninterrupted insurance coverage would result in large reductions in morbidity and mortality. Health care policy efforts aimed at increasing consistent insurance coverage in vulnerable populations are warranted.