We studied the serum Gm allotype of 58 patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and 236 nonrelated normal controls in Japan. The incidences of the Gm phenotype and haplotype in the patients were not significantly different from those in the normal controls. When CIDP was classified into two subgroups in terms of clinical course, the chronic relapsing (CR) and chronic nonrelapsing type (CNR), the distribution of all Gm haplotypes was significantly different between CR and CNR (total chi 2 = 8.319; corrected p < 0.05). Our findings suggest that the clinical course of CIDP may be associated with the Gm haplotype.