Some patients fulfilling the criteria for the diagnosis of multifocal motor neuropathy with conduction block (MMN-CB) at the onset of disease may subsequently develop a sensory loss associated with electrophysiological sensory abnormalities. The latter could represent an overlap between MMN-CB and multifocal acquired demyelinating sensory and motor (MADSAM) neuropathy. The objective was to specify the features of MMN-CB with sensory loss (MMN-CB-Se). Five patients in a series of 11 consecutive patients who fulfilled the criteria of the American Association of Neuromuscular and Electrodiagnostic Medicine for MMN-CB at the first examination and were treated periodically with intravenous immunoglobulin (IVIg) developed sensory loss in the course of the disease. In these five patients we compared the clinical, laboratory, and electrophysiological features found after the development of sensory loss with those at the first examination. The mean time to appearance of objective sensory signs was 7.2 years. In three of the five patients the sensory loss was preceded by intermittent paresthesias in the same nerve territories as the motor involvement. The most frequent electrophysiological abnormality was amplitude reduction of sensory nerve action potentials. There were no bilateral or symmetrical clinical and electrophysiological sensory abnormalities. Anti-GM1 IgM antibodies were positive in four patients. MMN-CB-Se could be an overlap between MMN-CB and MADSAM. It shares the distribution of the sensory disorders encountered in MADSAM, but it is closer to MMN-CB on clinical and therapeutic levels. Study of more patients would be useful to classify this subgroup more accurately.