In 51 cases (6 cases with bilateral operations) with various kinds of tremor, stereotaxic ventralis intermedius (Vim) thalamotomies were performed using Leksell's apparatus and the results of operation evaluated. Several characteristics of the tremor, including clinical features and EMG, were correlated with the assumed location and volume of the coagulative lesion. In 54 of the 57 operations, the thalamic Vim nucleus was identified physiologically and a therapeutic lesion placed at a site that included the Vim neurons. In all these cases, except one in which the lesion was estimated to be too small, tremor was immediately abolished by a relatively small lesion. The estimated volume of the lesion was about 40 to 200 mm3 and the effect persisted over a long follow-up period (maximum ten years). The size of the lesion that was necessary apparently depended on several features of the tremor. A larger lesion was required in cases of movement type tremor, tremor with a low rate (less than 4 Hz), tremor of high amplitude (more than 600 microV), and tremor involving proximal muscles or with a wide distribution. Tremor following a cerebrovascular lesion and post-traumatic tremor were characterized by coarse oscillation (high amplitude and low frequency) involving proximal muscles. A relatively larger coagulative lesion was therefore necessary to relieve this type of tremor. In contrast, parkinsonian and essential tremor were usually of low amplitude and distal in distribution. For the relief of such tremor, the lesion could be very small: if aided by electrophysiological methods to identify Vim neurons, the minimal effective volume of the lesion was estimated as about 40 mm3 and restricted to the Vim nucleus. Based on these results, the importance of the Vim nucleus in tremor mechanisms is discussed.