We have developed a frameless stereotactic multi-articulated arm suitable for image-directed microsurgery and surgical navigation. The system comprises a frameless isocentric mechanism and a three-dimensional (3 D) digitizer for intra-operative monitoring. Five joints of our multi-articulated arm are equipped with electromagnetic clutches, and the arms are provided with counterbalancing weights to ensure safe, smooth movement. Stereotactic microsurgical procedures were carried out with a laser beam guide mounted at the tip of the isocentric arm. We have used this system in 22 cases to resect tumours or epileptic foci. There were 12 cases of glioma, 5 cases of deep seated benign tumours, 3 cases of multiple metastatic tumours, and 2 cases of temporal lobe epilepsy. Postoperative enhanced CT images indicated total or subtotal resection in all cases. We also have used this system to determine the approach for clipping of a posterior cerebral artery aneurysm in 2 cases. The error of mechanical accuracy of the system was less than 1 mm. We believe that this system will prove useful in image-directed microsurgical procedures.