This study aimed to evaluate the influence of cognitive demand on prospective-memory in individuals with traumatic brain injury (TBI) using a dual-task paradigm. Fourteen individuals with severe TBI and 14 matched controls were required to undertake two tasks. A lexical-decision task was used as an ongoing task and had two levels of cognitive demand (viz., low and high). The event-based prospective-memory task involved performing a specific action whenever a target stimulus appeared during the ongoing task. The Letter-Number Sequencing Test, the Tower of London and the Controlled Oral Word Association Test were also administered to assess the relationship between prefrontal lobe functions and prospective memory. As hypothesised, participants in the TBI group performed more poorly than participants in the control group on the prospective-memory task in the high but not in the low demand condition. There were significant correlations between prospective-memory task performance and scores on the Letter-Number Sequencing Test and the Controlled Oral Word Association Test. These findings help to clarify the nature of prospective-memory impairment in individuals with TBI and support the involvement of prefrontal processes in prospective remembering.