We evaluated motor conduction studies in 129 patients with Charcot-Marie-Tooth disease type 1 (CMT1) to assess the uniformity of conduction slowing within individual patients. Conduction velocities were nearly identical in proximal and distal nerve segments (r = 0.86), from side to side (r = 0.95), in ipsilateral median and ulnar nerves (r = 0.941, and even in the median and peroneal nerves (r = 0.70). Segmental amplitude reductions suggestive of possible conduction block or differential dispersion were present in only 19 of 360 nerve segments studied (5.3%), and interphase cancellation or focal compression were likely alternative explanations in these cases. These findings support the concept that uniform conduction slowing characterizes CMT1, in distinction to acquired demyelinating polyneuropathies, which feature multifocal conduction abnormalities. We conclude that normal nerve conduction velocity in a single motor nerve can reliably exclude CMT1, but evaluation of several nerves and nerve segments to establish uniformity of conduction slowing is important in making this diagnosis. The homogeneity of the disturbance of nerve conduction favors a primary Schwann-cell disorder as the basis of CMT1.