Lewy body dementia (DLB), which share many clinical and pathological features with Parkinson’s disease (PD), is charac- terised by marked fluctuations in cognition and consciousness. Fluctuating cognition has not been formally studied in PD, although some studies indicate that PD patients show great variability in performance in reactiontime tasks. Aiming to investigate fluctuation of attention in PD, we re- analysed data from a cue-target reactiontime task, specifically searching for differences in variability between patients and controls. The subjects included were a representative group of PD patients (N484), and normal controls (N435). PD patients showed a considerably larger degree of variation in reaction time (RT), as evidenced by larger standard deviations (SD), than the control group (p< .001). A comparison of the coeffi- cient of variation between groups also revealed a significant difference (p<.001), indicating that differences in mean RT between groups cannot explain the larger SD’s in the patient group. This finding suggests that attention might be fluctuating on a moment to moment basis in PD. Some of the PD patients have also been tested with a choice reaction time task, shown by Walker et al. (2000) to be sensi- tive to fluctuation of cognition in DLB patients. Preliminary data-analysis indicate that PD patients also show considerable intra-individual variation in performance on this test. These findings suggest that fluctuating attention and cogni- tion is characteristic of PD, and that these fluctuations may be contributing to other cognitive deficits observed in this syn- drome. They provide new clinical information in PD, and add to other findings of clinical similarities between DLB and PD