Idiom interpretation tasks are routinely used in the clinical evaluation of adults with brain damage, and idiom processing has received increasing attention in the psycholinguistic literature. Clinical evidence suggests that adults with unilateral right-hemisphere damage (RHD) are insensitive to nonliteral meanings conveyed by idiomatic expressions and other figurative forms. However, this portrayal is derived from their terminal responses to tasks that reflect multiple aspects of mental operations (off-line measures), obscuring the source of poor performance. This study used an on-line word-monitoring task to assess RHD, left-hemisphere-damaged, and normally aging adults' implicit knowledge of familiar idiomatic expressions. Brain-damaged subjects performed similarly to normal controls on this task, even though the clinical subjects fared poorly by comparison on an off-line idiom definition measure. These results suggest that adults with unilateral brain damage can activate and retrieve familiar idiomatic forms, and that their idiom-interpretation deficits most likely reflect impairment at some later stage of information processing. Further, error analysis of idiom-definition performance did not support the customary characterization of RHD adults as excessively literal responders The paper discusses clinical implications of the nature and use of idiom interpretation tasks. (Connie A. Tompkins, Richard Boada, and Kathrine McGarry)