Finger tremor was investigated in 20 patients (age range 54-88 yr) diagnosed as suffering from idiopathic Parkinson's disease and six controls of a similar age and no known neurological abnormality. In nine of the patients tremor was not clinically obvious. When the tremor of these patients was recorded immediately after voluntary movement and subjected to instrumental analysis there were consistently observable differences from the controls. Such analysis may have diagnostic potential when there is clinical uncertainty. Surface EMG recordings were obtained from four patients. One patient had a large resting tremor with obvious reciprocating activity in flexors and extensors; in the others who had no symptomatic tremor there was reciprocating activity only after movement, and this died away in a few seconds as the induced tremor disappeared.