One theory for the pattern of cognitive impairment associated with Parkinson's disease is that deficits are apparent only on tasks which rely on the internal control of attention, which in turn is assumed to place heavy demands on limited attentional resources. This theory is evaluated in the present study by examining the performance of Parkinson's disease subjects and matched healthy control subjects on a series of fluency tasks. In one condition, subjects were required to generate exemplars under standard verbal fluency instructions, i.e. either to a single letter or a single semantic category. In a related condition, subjects had to generate exemplars alternately using two fluency probes, which could either be from the same domain, i.e. letter-letter or category-category, or from different domains, i.e. category-letter. The requirement to alternate between fluency probes presumably places greater demands on internal attentional control mechanisms, and in a yoked condition, subjects were given external cue cards to minimize these extra demands. Overall, there was no evidence that Parkinson's disease subjects had a basic fluency deficit, nor were they impaired when required to alternate between probes of the same domain. A specific deficit did emerge, however, for the mixed alternating condition (i.e. category-letter), in which Parkinson's disease subjects produced significantly more perseverative errors and had a significantly lower output. The former of these was reversible by the provision of external cue cards, but the overall output was unchanged. These data do not support the theory of internal attentional control, at least as a general theory of cognitive impairment in Parkinson's disease. To explain the observed pattern, we propose that Parkinson's disease is associated with: (i) a deficit in inhibitory attentional processes, and (ii) an impairment in the maintenance of those internal representations which control action.