The interplay between gait and specific cognitive faculties, in particular executive function (EF) and dual tasking abilities, has been described in healthy adults and in patients with Parkinson's disease (PD). There is, however, little direct evidence on the relationship between cognitive function, gait, and fall risk in PD, especially in the "ON" state (i.e., under the influence of the anti-parkinsonian medications). To address this issue, we evaluated cognitive function and gait under usual walking and dual-task conditions in 30 patients with PD in the ON state of the medication cycle. Subjects were classified as fallers or non-fallers based on their history. A computerized battery quantified cognitive function. Gait was assessed under three conditions: (1) Usual walking, (2) While subtracting serial 3 s, and (3) While subtracting serial 7 s. The EF and attention scores were lower in the fallers, compared to non-fallers (P ≤ 0.037), but general measures of cognition, e.g., memory, (P = 0.341) were not. Gait speed, variability, and the bilateral coordination of gait were worse in the fallers in all conditions. The DT effects on gait variability and bilateral coordination were larger in the fallers (P = 0.044, P = 0.061, respectively). These results suggest that patients with PD who have a high risk of falling are more sensitive to DT effects, perhaps as a result of relatively poor EF. These cognitive and motor deficits may increase the likelihood of loss of balance during everyday attention-demanding tasks among patients with PD.