This study was designed to investigate anorectal function in Parkinson's disease and multiple system atrophy (MSA). After a standardized interview, 17 patients with Parkinson's disease (PD) and 16 patients with multiple system atrophy (MSA) underwent anorectal manometry with a continuously perfused multi-lumen catheter, located to record pressures from the anal canal, and a balloon for rectal distension. Data were analyzed by observers blind to the neurologic diagnosis. Disease duration was shorter in the MSA than in the PD group (6 + 4 versus 10 + 5 yrs, p <0.05). Most patients reported a bowel frequency of less than three evacuations per week and some patients had fecal incontinence. Most manometric recordings disclosed an abnormal pattern during straining (a paradoxic contraction or lack of inhibition) in 13 patients with MSA and 11 patients with PD. Mean anal pressures and rectal sensitivity threshold were not significantly higher in the MSA group, whereas the inhibitory anal reflex and rectal compliance thresholds were within the normal range in both groups. Manometric patterns did not differentiate patients with MSA from patients with PD. Most patients in both groups showed an abnormal straining pattern, decreased anal tone, or both dysfunctions. In conclusion, our findings suggest that although bowel and anorectal dysfunctions do not differentiate MSA from PD, both abnormalities occur earlier and develop faster in MSA than in PD.