Letter fluency deficits are commonly detected in non-demented Parkinson's disease (PD) patients but the underlying cause remains uncertain. We investigated the role of slowed processing speed and executive dysfunction. Eighteen non-demented PD participants and nineteen controls were compared on letter fluency using a fluency index (Fi); the average time to “think” of each word, a measure independent of motor speed. Video analyses produced thinking times to switch between word clusters and generate a word within a cluster. Correlational and regression analyses were undertaken with tests of processing speed and executive functioning. The PD group exhibited significantly longer fluency indices than controls across all components. Performance on tests of executive functioning explained a significant proportion of variance whereas performance in processing speed tests did not. Moreover, PD participants with an executive functioning impairment showed significantly worse switching fluency indices only compared with Controls and PD participants without executive dysfunction. PD participants with executive dysfunction exhibited a disproportionate impairment in the time taken to switch between clusters than to think of words within clusters. Executive functioning contributed to fluency performance more than processing speed. Cognitive heterogeneity and motor slowing, may mask the profile of cognitive dysfunction in neurodegenerative disease. (JINS, 2013, 19, 1–9)