The frontal assessment battery (FAB) is a bedside test of executive function. It takes less than 10 minutes to administer and a low score indicates executive dysfunction. To determine whether the FAB could detect the more severe subcortical dementia that is a feature of PSP and differentiate it from other bradykinetic rigid syndromes, we studied 17 patients with progressive supranuclear palsy (PSP); 11 with multiple system atrophy (MSA) and 12 with Parkinson's disease (PD). We compared FAB scores with the results of more detailed tests of executive and general cognitive function.FAB scores were significantly lower in PSP than in MSA or PD (p=0.02 and p<0.001) and were also found to be significantly lower in MSA than in PD (p=0.047). We divided the study group into those with an FAB score <15 and those with an FAB score>/=5, regardless of the clinical diagnosis. While 82% of the PSP group had FAB scores of <15, such scores were recorded in only 36% of the MSA and 8% of the PD groups. The lexical fluency and motor series subscores of the FAB discriminated 70% of the PSP, MSA and PD patients. The FAB scores correlated with tests of executive function, as well as with scores on the Mattis Dementia Rating Scale, the Mini Mental State Examination and other tests of general cognitive function. A stepwise regression analysis revealed that across the groups, among the variables that correlated with FAB scores, alternating semantic fluency accounted for 80% of FAB variance.These results suggest that the FAB is a valid and easily applicable bedside test to discriminate executive dysfunction in these three frequently confused bradykinetic rigid syndromes.