RNA-RNA tissue in situ hybridization is a relatively new technique that detects gene expression in individual cells. In this report we compare and contrast the technique with conventional biologic analysis. We illustrate how this technique could function as a diagnostic tool by applying it to a 58-year-old man with a four-month history of lymphadenopathy and peripheral lymphocytosis. RNA-RNA tissue in situ hybridization performed on sections of one of this patient's lymph nodes and on cytospins of his peripheral blood demonstrated the presence of an apparent monoclonal population of B cells producing mu and lambda immunoglobulin (Ig) messages in the lymph node and peripheral blood as well as a T-cell population in the lymph node only. These results were corroborative and complementary to conventional DNA (Southern) and RNA (Northern) analyses. The data were consistent with the diagnosis of chronic lymphocytic leukemia (CLL). With the use of this technique, an intriguing pattern of cellular heterogeneity was observed within the mu-lambda population of cells in the lymph node. A subset of these cells appeared to express a much greater amount of immunoglobulin message and to cluster around the lymph node vessels. The combination of RNA-RNA in situ hybridization and routine histopathology has the potential for providing an additional dimension to tumor analysis.