<i>Background:</i> Spatial function has been suggested to be disproportionately worse in people with dementia with Lewy bodies (DLB) than other dementia groups, and poor performance on the Mini-Mental State Examination pentagon copying (PC) task has been proposed as adequate for assessing this. We aimed to establish the prevalence of poor PC in the non-demented elderly; determine the validity of the use of PC as a spatial function test, and determine if poor PC is more common in DLB than non-DLB dementias. <i>Methods:</i> In a population-based sample of 299 participants, 126 were rated as being cognitively normal (clinical rating scale [CDR] = 0), 95 mildly cognitively impaired (CDR = 0.5), and 78 met criteria for dementia, 19 of whom met criteria for probable DLB (pDLB) and 25 with none of the core features of DLB (non-DLB). The accuracy of PC performance was determined across CDR groups, and the relationship of PC to performance on a broad range of cognitive tests was evaluated. The dementia groups were compared cross-sectionally to determine differences in PC and other cognitive test performance, as well as 3 and 6 years earlier to determine cognitive differences at initial stages of cognitive decline. <i>Results:</i> Poor PC was common in the non-demented elderly (39% CDR = 0; 43% CDR = 0.5). In this non-demented group, PC was selectively related to tests of spatial function. Poor PC was not significantly different in the pDLB and non-DLB groups at any assessment time, however it became more prevalent as dementia severity increased. Memory function and verbal fluency were more impaired in the pDLB group in the early stages of the disorder. <i>Comment:</i> PC appears to be a good measure of spatial function in the elderly. However, in contrast to other findings of poor spatial skills in DLB when dementia is in the mild to moderate stages, poor PC performance has not been shown to be a good early marker of DLB and its clinical correlates are yet to be determined.