We have analyzed the immunoglobulin heavy chain (VH) gene variable regions (CDR2 and FW3) of 101 Japanese cases with peripheral B cell neoplasms. When all except one case with a deletion were graphed by frequency of replacement mutation, the 100 cases could be separated into two groups: 24 cases with zero, one and two mutations (germline or low frequency of somatic mutation); and 76 cases with three or more mutations (medium to high frequency of somatic mutation). While most mantle cell lymphoma cases (11/13) showed germline or low frequency of somatic mutation, all cases of mucosa-associated lymphoid tissue (MALT) lymphoma (11/11), follicular lymphoma (three of three cases), plasma cell myeloma (seven of seven cases) and most cases of diffuse large B cell lymphoma (DLBCL; 42/47) belonged to the latter group. These 76 cases, therefore, may be considered to show somatic hypermutation. More than half of chronic lymphocytic leukemia/small lymphocytic lymphoma cases (CLL/SLL; eight of 13) showed a hypermutated VH gene and the ratio of replacement mutation: silent mutation in CDR2 of CLL/SLL was considerably higher compared with DLBCL and MALT lymphoma, showing somatic hypermutation. When comparing VH gene type of B cell-CLL (B-CLL) among our series and those in the literature, more cases of CD5+ B-CLL in the Western literature have the VH5 and VH6 family types, while more cases in Japan are reported to have VH4 family. The occurrence of VH families in B-CLL between Japanese and Western people seems to be comparable.