Bradykinetic-rigid syndromes are often accompanied by cognitive impairment. Because prominent motor involvement in these disorders may interfere with neuropsychological testing, we used event-related potentials (ERPs) for the assessment of cognition and attention in 41 patients with various bradykinetic-rigid syndromes of less than 5 years duration: idiopathic Parkinson's disease corticobasal degeneration, Steele-Richardson-Olszewski syndrome (SRO), and multiple system atrophy. Patients were compared with matched normals. ERP abnormalities in the auditory "oddball" paradigm were found only in corticobasal degeneration and SRO. ERP abnormalities in selective attention tasks were present in all patient groups, changes in SRO being the most prevalent. Abnormalities in corticobasal degeneration were present under "less-attention-demanding" conditions and suggested involvement of posterior parts of the brain. Multiple system atrophy and idiopathic Parkinson's disease patient groups had minimal ERP abnormalities. However, reaction times in MSA were longer in all paradigms. The results of the study support the view that bradykinetic-rigid syndromes involve some attentional deficits, but also have distinct reaction time and ERP characteristics, which may be helpful in differential diagnosis.