Using PCR method, we analyzed the gene rearrangement of T-cell receptor chain (TCR-beta) in 14 cases of midline malignant reticulosis (MMR). Our results showed that there were non-germline, monoclonal rearrangements of TCR-beta in 12 of 14 cases of MMR (85.7%). In contrast, the same rearrangement pattern was found only in 6 of 10 cases of lymph node based T-cell lymphoma. The cases of reactive lymphoid hyperplasia were of a polyclonal rearrangement pattern. This study suggests that MMR is a monoclonal T-cell neoplasm and PCR method is a rapid, facilitated and useful method for the detection of clonalities of T-cell proliferations.