Two experiments were conducted to examine the effects of task importance on event-based prospective memory (PM) in separate samples of adults with HIV-associated neurocognitive disorders (HAND) and HIV-infected young adults with substance use disorders (SUD). All participants completed three conditions of an ongoing lexical decision task: (1) without PM task requirements; (2) with PM task requirements that emphasized the importance of the ongoing task; and (3) with PM task requirements that emphasized the importance of the PM task. In both experiments, all HIV+ groups showed the expected increase in response costs to the ongoing task when the PM task’s importance was emphasized. In Experiment 1, individuals with HAND showed significantly lower PM accuracy as compared to HIV+ subjects without HAND when the importance of the ongoing task was emphasized, but improved significantly and no longer differed from HIV+ subjects without HAND when the PM task was emphasized. A similar pattern of findings emerged in Experiment 2, whereby HIV+ young adults with SUD (especially cannabis) showed significant improvements in PM accuracy when the PM task was emphasized. Findings suggest that both HAND and SUD may increase the amount of cognitive attentional resources that need to be allocated to support PM performance in persons living with HIV infection. (JINS, 2014, 21, 1–11)