Sonography of the bladder was performed before and after voluntary voiding in 20 subjects with possible/probable multiple system atrophy (MSA), 20 patients with idiopathic Parkinson's disease (PD), and 20 healthy controls. Mean residual volume was 173 ml (SD, 194) in MSA and significantly increased compared to both PD (21 ml; SD, 49) and controls (13 ml; SD, 12). Incomplete voiding (>100 ml of residual volume) was found in 11 subjects with MSA but only in 1 patient with PD. Positive predictive value of increased residual volume for MSA was 91.6% in this study, whereas negative predictive value was only 67.8. Bladder sonography is an objective, simple, and safe tool that allows one to screen for urinary retention, which is highly suggestive but incompletely sensitive for MSA. Because sonography is easily accessible and rapidly performed, it is feasible for routine assessment of atypical Parkinsonism.