Twenty-five patients with recently diagnosed leptomeningeal metastases underwent a prospective clinical and electro-physiological study to assess the mode and extent of peripheral nerve and spinal root involvement. Motor conduction velocities and F wave latencies in the legs were found frequently to be disturbed compared with normal subjects and a control group of cancer patients. F wave changes were the most sensitive indicator of root involvement. Electrophysiological abnormalities were detected in 20 patients, compared with 15 who had abnormal clinical signs suggesting peripheral nerve dysfunction. Electrophysiological studies can be useful in the evaluation of patients with suspected leptomeningeal metastases, and repeated recording of F wave latencies may serve to indicate root recovery in a clinically stable patient.