Objective-To study the process of denervation-reinnervation in multifocal motor neuropathy with persistent conduction blocks in clinically aVected and unaffected muscles. Method-Volitional single fibre electromyography (SFEMG) was performed in the extensor digitorum communis (EDC) of seven patients. The jitter, the fibre density, and the mean interpotential interval were determined. The results before and after treatment with intravenous immunoglobulin (IVIg) between the unaVected EDC and aVected EDC examined during the same SFEMG session were also compared. In addition the values of jitter, fibre density, and mean interpotential interval were analysed for correlation with the strength score on the MRC scale, the duration of the neuropathy, the number of IVIg treatment periods, and the radial nerve conduction block values. Results-Mean jitter, percentage of jitters>60 µs, and impulse blocking percentage, were higher than normal in both the aVected EDCs and to a lesser degree in unaVected EDCs. Jitter decreased significantly after IVIg and correlated only with the MRC score. Fibre density and mean interpotential interval were higher than normal equally in the aVected EDC and unaVected EDCs, but no correlation was found with strength, duration of the neuropathy, number of treatment periods, and conduction block values. Conclusion-The major finding is the presence of SFEMG abnormalities in clinically unaVected EDCs. This shows a process of denervation-reinnervation even in the absence of clinical symptoms, probably more frequent than commonly supposed in this neuropathy. The rapid clinical improvement after IVIg infusions could be due to remyelination after demyelination and to an interference of IVIg with the blocking eVect of antibodies on the Na + channels at the motor nerve endings. (J Neurol Neurosurg Psychiatry 1998;65:357-361)