We have been interested in the application of quantitative measures of motor performance as a possible means of early detection of Parkinson's disease. To assess motor function, we have measured movement time (the physiologic correlate of bradykinesia) and reaction time (simple and directional choice) with an upper limb motor task, and tremor with accelerometry and electromyographic recordings. In this report we describe preliminary data from a Parkinson's disease patient group with symptoms of fewer than 2 years' average duration (compared with an age- and gender-matched normal control group) which indicate that precise, quantitative tests of motor function can detect the slight deviations from normal that are present in early Parkinson's disease. It appears that tests of bradykinesia are most sensitive, and detection of rest tremor is most specific. These tests may be applicable in screening individuals who are suspected of having or are "at risk for" Parkinson's disease and other related disorders.