It is clinically important, to distinguish between idiopathic Parkinson's disease (IPD) and multiple system atrophy (MSA) not only because of the implications for prognosis but also because urinary incontinence is often an early troublesome feature of MSA and by making the correct neurological diagnosis inappropriate urological surgery may be avoided. Onuf's nucleus in the sacral cord is the location of the anterior horn cells innervating the sphincters, and it is among central nervous system sites affected by neuronal cell loss in MSA but not in IPD. A systematic analysis of motor units recorded from the sphincter looking for changes of chronic reinnervation has therefore been used to distinguish between these conditions. Sphincter electromyography (EMG) was carried out in 126 patients with suspected MSA with review of their case notes up to 2 years later. Of those in whom a diagnosis of MSA was made, 82% had had an abnormal sphincter EMG.