The sustained attention of young adults with mild mental retardation and those without retardation was assessed during three 50-minute tests in which visual cues were presented successively on a computer-controlled monitor. Target cues occurred on 5% of the stimulus presentations in a random fashion. Significant decrements in target detection were observed in both groups of observers during all tests; however, the vigilance decrement was unrelated to intelligence level. The target-detection ability of observers with and without mental retardation did not differ on a recognition task. The observers with mental retardation detected significantly fewer target events than did observers who were not retarded on two tasks requiring judgments to be made on the basis of a remembered standard. These data suggest the sustained attention of adults with mental retardation differs from that of adults without mental retardation on vigilance tasks that place demands on memory abilities.