Nonsyndromic low-frequency sensorineural hearing loss (LFSNHL) comprises a group (DFNA1, DFNA6, DFNA14, and DFNA38) of hearing disorders affecting only frequencies below 2000 Hz, and is often associated with tinnitus. An LFSNHL locus has recently been assigned to chromosome 4p16, and mutations in WFS1, the causative gene for Wolfram syndrome, have been found to cause LFSNHL in families with DFNA6, DFNA14, or DFNA38. We performed a genome-wide linkage analysis of a Japanese family in which 20 members were affected with LFSNHL and obtained a maximum LOD score of 5.36 at a recombination fraction of 0.05 ( P = 1.00) at the D4S2983 locus on 4p16. Haplotype analysis revealed that the disease locus mapped to between D4S2366 and D4S2983. Mutation analysis revealed a novel missense mutation (K634T) in WFS1. We thus concluded that the LFSNHL in this family was caused by the WFS1 mutation. The mutation observed (K634T) was located in the hydrophobic, extracytoplasmic, juxta-transmembrane region of the WFS1 protein, wolframin, and was hitherto undescribed. This unique mutation site in our patients is likely related to their milder phenotype (lacking tinnitus) compared with those of six previous DFNA6/14 patients with WFS1mutations. It is likely that a genotype-phenotype correlation is also applicable in the case of DFNA6/14/38.