Diagnosis of multifocal motor neuropathy (MMN), a syndrome characterized by progressive asymmetric weakness with intact sensation, is important because the disorder often responds to treatment. Multifocal partial motor conduction block (PMCB) has been emphasized as a cardinal feature in the diagnosis of this syndrome, but detailed nerve conduction studies are not available. Nine patients, ages 28-58, had chronic, progressive, asymmetric, predominantly distal limb weakness for 5-18 years. Sensation was normal and reflexes were reduced asymmetrically. Although all 9 demonstrated PMCB localized to short nerve segments, additional features of multifocal motor demyelination were present, including temporal dispersion (5 patients), segmentally reduced motor nerve conduction velocity (7 patients), prolonged distal motor latency (4 patients), and prolonged F-wave latency (9 patients). The strength of all patients improved after treatment with human immune globulin. A reduction in the degree of PMCB or an increase in the distal motor amplitude or both accompanied the clinical improvement. These studies suggest that patients with MMN demonstrate widespread evidence of motor demyelination in addition to the well-described PMCB, and that reduction of PMCB accounts for the increase in strength following therapy.