BACKGROUND Low grade, small lymphocytic, non-Hodgkin's lymphoma, was diagnosed in a 38-year-old woman. Thirty months after the initial diagnosis was made, a population of lymphoid cells with pathologic morphology was found in the patient's peripheral blood (PB). Cell phenotyping was performed and monoclonality was analyzed in cells obtained from a removed lymph node (LN) and the PB of the patient. METHODS The cell phenotype was examined with immunofluorescence techniques using antibodies against SIg and monoclonal antibodies against CD1, CD3, CD4, CD5, CD8, CD19, and the kappa, and lambda light chains. Gene rearrangement analysis for monoclonality determination was performed with restricted DNA (EcoRI, Hin-dIII and BamHI) hybridized with either 32P-labeled T-cell receptor DNA probe (TcR-beta) or immunoglobulin-heavy chain probe (JH). RESULTS With regard to the cell population of the removed LN, cell phenotyping showed the predominance of CD4+ T-cells over a polyclonal B-cell population. Gene rearrangement analysis proved the monoclonal nature of the T-cells and the polyclonal nature of the B-cells. As to the PB, gene rearrangement and cell phenotyping of the lymphocytes showed the predominance of monoclonal kappa type B-cells over polyclonal T-cells. CONCLUSIONS The data obtained suggest two unrelated lymphoproliferative diseases in this patient, expressed as monoclonal T-cell population in LN and as monoclonal B-cell population in PB.