T cell large granular lymphocyte leukemia (T-LGL) is characterised by semiautonomous proliferation of monoclonal cytotoxic T lymphocytes, which can result in neutropenia, splenomegaly, and is associated with various autoimmune disorders, particularly rheumatoid arthritis. The coexistence of T-LGL leukemia with B cell abnormalities has previously been identified in case reports. However, no systematic analysis to determine the frequency of this co-association has been reported. Analysis of 63 T-LGL patients revealed a frequent association with humoral immune system abnormalities. We identified coexisting B cell dyscrasias in 17 T-LGL patients (27% of total), of whom 12 had monoclonal gammopathy of unknown significance (MGUS) (19%), and 5 had chronic lymphocytic leukemia (CLL) (8%). The presence of both MGUS and CLL was found in 2 patients (3%) and follicular lymphoma was identified with MGUS in another T-LGL patient (2%). Additionally, polyclonal hypergammaglobulinemia or hypogammaglobulinemia was found in 10 additional LGL leukemia patients bringing the total frequency of B cell abnormalities in T-LGL leukemia to 43% in our cohort. The co-association of B cell pathology with T-LGL suggests that either a common antigen drives clonal B and T cells, or that humoral malignancy could serve as the stimulus for lymphocyte expansion representing an overactive anti-tumour surveillance.