Background: The flow cytometric analysis of surface immunoglobulin light chains (sIgL) is used as a simple method for evaluating monoclonal B-cell proliferation. However, the sIgL expression, kappa or lambda, is occasionally undetectable in cases with B-cell non-Hodgkin lymphomas (B-NHL). The purpose of this study was to investigate the clinical and pathological characteristics of these B-NHL cases. Methods: We retrospectively analyzed 50 cases with previously untreated sIgL-negative B-NHL. All of these cases had been diagnosed at Tokai University Hospital between January 2001 and February 2011. Their medical charts were reviewed. Results: These cases had several clinical features: diffuse large B-cell lymphoma (DLBCL) (72%), a high serum lactate dehydrogenase level (66%), clinical stage III and IV (68%), and complex karyotypes (58%). Seven out of eight evaluated patients (87%) did not express cytoplasmic IgL, and the DNA rearrangement pattern of IgL showed diversity in 10 analyzed patients. The 5-year event-free survival of all the sIgL-negative B-NHL cases was significantly better with rituximab-containing chemotherapies in comparison to the regimens without it (57.9% vs. 17.9%, p=0.0207), although there was no statistical significance when the DLBCL cases were analyzed (56.6% vs. 22.2%, p=0.1530). Conclusions: These findings suggest that sIgL-negative B-NHL cases predominantly developed DLBCL in advanced disease, but were heterogeneous at the molecular level.