Ska and colleagues (1990) reported that patients with possible or probable Alzheimer's Disease (AD) make characteristic errors on a measure of line orientation judgment. We examined errors of line orientation judgment in a sample of 31 patients with probable AD, 28 patients with idiopathic Parkinson's disease (PD), and 24 normal elderly controls (NC). Results indicated that, in comparison to NC participants, some error types were made more frequently by patients with PD and patients with AD. No error type, however, was specific to AD alone. in contrast, PD patients were more likely than AD or NC participants to make a specific error in oblique line judgment. This finding is discussed in the context of visuospatial analysis and selective attention in patients with PD.