We analyzed nucleotide sequence and intraclonal diversity of the rearranged immunoglobulin heavy chain gene variable region (VH gene) of CD5+ and CD5- diffuse large B cell lymphoma (DLBCL) to clarify the cell origin of de novo CD5+ DLBCL. Ten cases of CD5+ DLBCL and 29 cases of CD5- DLBCL were analyzed. The frequencies of somatic mutation were 0.7 to 12.9% (average, 6.2%) in CD5+ DLBCL and 2.0 to 25.9% (average, 11.1%) in CD5- DLBCL. The ongoing mutation rate was estimated from the number of further single base-substitutions, expressed as a percentage of the total number of nucleotides in 10 cloned PCR products for each case (%). The averages of the ongoing mutation rate of CD5+ DLBCL (four cases) and CD5 DLBCL (seven cases) were 0.051% and 0.197%, respectively. The rate of CD5+ DLBCL was significantly lower than that of CD5- DLBCL (t-test, P = 0.024). These data may indicate that the cell origin of CD5+ DLBCL is different from that of CD5- DLBCL. CD5 is not an activated antigen in DLBCL, but a specific marker of the B1 subset of the B cells, and de novo CD5+ DLBCL may therefore be derived from this unique subset.