Patients with Parkinson's disease (PD) can experience cognitive impairment. There are currently no medications indicated for the treatment of cognitive impairment in PD. Clinicians are faced with the dilemma as to whether or not to treat patients with PD with the acetylcholinesterase inhibitors that are currently approved for use in Alzheimer's disease (AD) and that have shown promise in clinical trials of Dementia with Lewy bodies (DLB). Although these medications may help cognition, there is a theoretical concern that by increasing acetylcholine relative to dopamine, they might worsen motor function. We report the case of a patient with PD and cognitive impairment who developed a marked worsening of motor function, mood, and anxiety in the setting of a pharmacologic challenge study using a 3-mg oral dose of the acetylcholinesterase inhibitor, rivastigmine. We believe that the mechanism of the motor and perhaps psychiatric worsening was increased central cholinergic tone. We conclude that further studies should be done to evaluate the efficacy and tolerability of these agents in this illness but that caution should be exercised when using acetylcholinesterase inhibitors in patients with PD.