Chronic lymphocytic leukemia (CLL) is relatively rare in Japan. Among 46 cases of mature B-cell leukemia, we identified 28 Japanese patients with CLL, including prolymphocytoid and lymphoplasmacytoid morphological variants. Compared with Western patients with CLL, only 52.0% of cases showed the typical immunophenotypic profile. IgG-bearing (15.4%) and clearly CD20-expressing (71.4%) cases were frequently observed. Most cases harbored a mutated immunoglobulin heavy-chain (VH) gene (88.5%) and commonly used a VH3 family member (61.5%) other than VH3-21. During the median follow-up period of 64 months, 20 cases (71.4%) showed an indolent clinical course without any treatment, and six cases (21.4%) were accompanied by other malignancies. Binet A stage (p = 0.003), low-risk category according to the modified Rai classification (p = 0.016), and ≤ 15 U/mL level of serum thymidine kinase activity (p = 0.016) were associated with prolongation of treatment-free status. Although Japanese cases of CLL showed heterogeneity in morphology and immunophenotype, most cases arose from post-antigen-selected B cells and presented with indolent clinical behavior.