BACKGROUND: One third of patients with epilepsy fail to gain optimal control using antiepileptic drugs. New advances in epilepsy surgery have reshaped some diagnostic and therapeutic modalities into less invasive approaches. To understand the cortical epileptogenic networks, stereoelectroencephalography uses depth electrodes as a tool for invasive intracranial monitoring. These electrodes are now being placed using a robot instead of the traditional Talairach stereotactic grid and frame with comparable accuracy and the advantage of shortened surgical time. Whereas accuracy is important for correct cortical sampling and surgical morbidity, the factors that play a role in the deviation of the electrode's trajectory are not yet understood. The aim of this study was to determine if the angle the planned trajectory makes with the skull impacts the final accuracy of placement. METHODS: Twenty-five patients with 319 stereotactic trajectories were examined. Skew angle and accuracy measurements were computed based on preoperative and postoperative computed tomography scans fused with trajectories planned using ROSA robot software. RESULTS: Trajectories with a planned angle of >30° (oblique group) had significantly higher radial error (2.05 mm vs. 1.45 mm, P < 0.001) than trajectories with planned angles <30° (orthogonal group). CONCLUSIONS: The ROSA robotic system is a powerful tool that increases the speed and efficiency of stereotactic neurosurgery. Care should be taken when planning trajectories to avoid high skew angles. If a high skew angle is unavoidable, care should be taken to ensure a larger margin of safety.