The current study evaluated the effects of disease severity on the control of obstacle crossing in people with idiopathic Parkinson's disease (PD). Forty-five subjects participated in the study, including 15 patients with mild PD (classified as stage 1 to 1.5 of the Hoehn and Yahr Rating Scale), 15 patients with moderate PD (classified as stage 2 to 3 of the Hoehn and Yahr Rating Scale), and 15 neurologically healthy individuals. Groups were matched by sex, age, body mass, and body height. The obstacle crossing task required participants to walk along a pathway and step over an obstacle (half of the knee height, positioned in the middle of the pathway). Patients were tested in a typically medicated state. Kinematic data were recorded using an optoelectronic tridimensional system. The outcome measures included spatiotemporal measures of obstacle avoidance. There were no significant differences between patients with mild PD and healthy individuals. Patients with moderate PD exhibited shorter distances for leading toe clearance and leading foot placement after the obstacle than did healthy individuals. Patients with moderate PD tended to exhibit a lower leading horizontal mean velocity during obstacle crossing than did healthy individuals. We found significant negative relationships between obstacle crossing measures and disease severity (score on the motor section of the Unified Parkinson's Disease Rating Scale). These findings suggest that disease severity affects locomotor behavior during obstacle crossing in PD. Specifically, obstacle avoidance was not affected in the early stages of PD; however, bradykinesia and hypometria influenced obstacle crossing in patients with moderate PD.