Monkeys with lesions of the mid-dorsal part of the lateral frontal cortex, which extends above the sulcus principalis as far as the midline (MDL lesions), were shown to exhibit severe and long-lasting impairments on certain nonspatial working memory tasks: the self- ordered and externally ordered tasks (experiments 1, 2, 3, 5, 6, and 8). These tasks, which were modeled on similar ones previously used with patients, measure the capacity to monitor, within working memory, self-generated choices and the occurrence of externally ordered stimuli. Monkeys with lesions of the adjacent posterior dorsolateral frontal cortex, which surrounds the arcuate sulcus (PA lesions), performed as well as the normal control animals on these tasks. Experiments 4 and 5 showed that the critical variable accounting for the impairment on the self-ordered and externally ordered working memory tasks by monkeys with MDL lesions is the size of the set of stimuli that must be monitored. Furthermore, the MDL lesions did not affect basic recognition memory (experiment 6), or primacy and recency mnemonic effects [i.e., the capacity to discriminate between the initial (or final) items and other items in a list of stimuli (experiments 4 and 7), or the capacity to select from a set of stimuli on the basis of a learned fixed sequence (experiment 9)]. Thus, lesions of the mid-dorsal part of the lateral frontal cortex give rise to an impairment in working memory that depends on the size of the set of the stimuli that have to be monitored.