Visual-constructional apraxia is a prominent feature of dementia with Lewy bodies (DLB) that might help to clinically distinguish it from Alzheimer's disease (AD). The main goal of this study was to assess performance on the copy intersecting-pentagon item of the Mini-Mental State Examination with the new Qualitative Scoring method for the Pentagon copy Test (QSPT). In order to determine which aspects of the drawings might differentiate DLB from AD, pentagon drawings of autopsy-verified DLB (n = 16) and AD (n = 15) patients were assessed using the QSPT. The qualitative scoring encompasses the assessment of different parameters of the drawing, such as number of angles, distance/intersection, closure/opening, rotation, and closing-in. The QSPT scores were compared between groups using linear analyses and artificial neural network analyses at four different time points. Linear analyses showed that during the first evaluation, number of angles was the only parameter that showed a significant difference between DLB and AD patients. A gradual decline in other parameters and total pentagon score occurred in both groups during subsequent years, with greater decline for the DLB group. The artificial neural network analyses using auto-contractive maps showed that, with disease progression, DLB became related to relatively lower qualitative pentagon scores, whereas AD became related to relatively higher qualitative scores. These findings suggest that the QSPT might be a sensitive measure of visuo-constructive abilities able to differentiate DLB from AD at disease onset and as the diseases progress, but further studies on larger population are necessary in order to establish its clinical relevance.