A case of small intestinal extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) with monoclonal cryoglobulinemia is described. The patient was a woman in her mid-sixties with purpura of the bilateral lower legs and abdominal pain. An immunoserological investigation showed expression of IgM-kappa type monoclonal cryoglobulin. A renal biopsy specimen revealed proliferative glomerulonephritis with cryoglobulin deposition. Physical examination disclosed a stenosis, edematous changes and ascariasis in the small intestine. In aspiration cytology of the ascites, proliferation of the atypical lymphoid cells with plasmacytoid differentiation was observed. These cells were positive for B-lineage antigens in immunocytochemistry, and showed an immunoglobulin heavy-chain gene rearrangement in Southern blotting and chromosomal alteration in G-banded karyotype analysis. Although medicinal treatment was used, the patient died of general prostration. The diagnosis of intestinal MALT lymphoma was made at autopsy. Expression of API2-MALT1 fusion transcripts was detected by reverse transcription-polymerase chain reaction analysis using formalin-fixed, paraffin-embedded tissue. Intestinal MALT lymphomas with API2-MALT1 expression have distinctive forms of infiltration compared with those without translocation. Therefore, detection of API2-MALT1 fusion transcripts is useful for evaluating the prognosis and clinical behavior of the disease.