A series of 54 patients presenting with parkinsonism underwent clinical assessment and urethral sphincter electromyography (EMG). After clinical assessment, 26 were thought to be suffering from probable multiple system atrophy (MSA), 15 were thought to have possible MSA and 13 were diagnosed as having probable idiopathic Parkinson's disease (IPD). Of those with probable MSA, 16 were found to have an abnormal urethral sphincter EMG. In the group with possible MSA, only 5 patients had an abnormal EMG while in the group with probable IPD, only 1 patient had an abnormal EMG. It was concluded that urethral sphincter electromyography provides a useful method of distinguishing between idiopathic Parkinson's disease and multiple system atrophy. It also provides a means of identifying those patients with parkinsonism whose incontinence may well be worsened, or in whom incontinence may develop following lower urinary tract surgery.