Arm tremor in Parkinson’s disease occurs during rest which has been explained by a specific tremulous mechanism activated during rest. During action or posture maintenance, the tremor may persist or recur, but with reduced amplitude. We present 2 patients with idiopathic Parkinson’s disease who showed persisting arm tremor which was most marked during action rather than during rest. The patterns of upper limb tremor were investigated during rest, maintenance of fixed posture, and slow movement, using an advanced computerized movement-monitoring system (Coda 3). Four parkinsonian patients showing tremor under all conditions were studied, comparing 2 patients whose tremor was most marked during rest with 2 whose tremor was most marked during action. All showed intermediate amplitudes during posture maintenance. Patterns of tremor frequency were very similar between patients, irrespective of amplitude characteristics. These features suggest that tremors in all patients originate in a single, complex mechanism which may involve both central and peripheral mechani