Despite the low incidence of microsatellite instability (MSI) in lymphoid malignant neoplasms, it has been reported that the CD5 promoter MSI was relatively frequent among B-cell chronic lymphoproliferative disorders. We studied the presence of MSI in the CD5 promoter in 134 cases of B-cell chronic lymphocytic leukemia (B-CLL) and 47 of mantle cell lymphoma (MCL) by comparing the pattern of microsatellite repeats on autologous germline and tumor DNA samples. Microsatellite alterations were not observed in any case. However, the allele distribution of this polymorphism showed a higher frequency of the 18 CA allele (0.585) in MCL cases (P = .026; odds ratio [OR], 1.75; 95% confidence interval [CI], 1.07–2.87) and of the 19 CA allele (0.179) in B-CLL cases (P = .005; OR, 2.26; 95% CI, 1.27–4.01) compared with control cases (0.442 and 0.087, respectively). This suggests that although MSI seems not to be involved in the pathogenesis of these 2 lymphoid malignant neoplasms, the polymorphic CD5 promoter is associated with increased susceptibility to these disorders.