Follicular lymphoma is categorized as low-grade lymphoma because of the long median survival, but the disease is difficult to cure because of frequent recurrence. The t(14;18)(q32;q21) associated with follicular lymphoma juxtaposes a portion of BCL-2 (18q21) and IGH(14q32); the result is bcl-2 overexpression. In this study, a highly sensitive 2-color fluorescence in situ hybridization (FISH) method was used to detect t(14;18)(q32;q21) in the nuclei of paraffin-embedded tissue sections. Fourteen specimens, including 11 samples from follicular lymphoma and 3 samples from diffuse large cell lymphoma, for which results of karyotype study and paraffin-embedded tissues were available were selected for FISH study. The FISH results were compared with results of karyotype study of the lymph nodes involved in lymphoma. Among our 11 patients with the diagnosis of follicular lymphoma in whom karyotype study was performed, 8 had t(14;18)(q32;q21) in karyotype analysis, and 7 of these patients had a positive pattern in FISH analysis. In 1 case, FISH analysis was difficult because of weak signals. All 3 patients with diffuse large cell lymphoma and t(14;18)(q32;q21) in karyotype analysis had a positive pattern in FISH analysis. In 3 cases of follicular lymphoma without t(14;18)(q32;q21) in karyotype analysis, FISH did not show a positive pattern. Therefore the FISH assay in tissue was found to be very sensitive in detection of IGH/BCL2 translocation and was helpful in diagnosis of follicular lymphoma or in clarification of the cell origin of lymphoma when karyotype analysis was not available. Performing FISH on paraffin sections also is useful because we can identify cells with genetic abnormalities in the tumor and make a retrospective cytogenetic diagnosis even with old paraffin-embedded specimens.