BACKGROUND: Falls are common in older people with Parkinson's disease (PD) and are likely to be related to gait disturbances associated with the condition. Although several studies have evaluated differences in basic gait parameters in people with PD, none have directly evaluated the stability of the upper body during gait. METHODS: Temporospatial gait parameters and acceleration patterns at the head and pelvis were measured in three groups of older people: 33 controls without PD (mean age 67 +/- 4 years), 33 older people with PD and no history of falls (mean age 63 +/- 4 years), and 33 older people with PD and a history of falls (mean age 67 +/- 2 years). Harmonic ratios of head and pelvis accelerations in each plane were calculated to provide an indicator of upper body stability. RESULTS: Compared with the control group, older people with PD exhibited significantly reduced walking speed and step length and increased step timing variability. Acceleration patterns were also significantly less rhythmic at the head and pelvis in all three planes. After adjusting for differences in walking speed and step timing variability, PD fallers exhibited significantly less rhythmic accelerations at the pelvis in the vertical and anteroposterior planes than PD nonfallers. CONCLUSIONS: Acceleration patterns during gait differ between older people with and without PD and between older people with PD who do and do not fall. These findings suggest that an inability to control displacements of the torso when walking may predispose older people with PD to falls.