The histologic and immunohistochemical findings in bone marrow (BM) biopsies from 38 patients with peripheral T-cell lymphoma (PTCL) are reported. Routine light microscopy showed that BM involvement was unequivocal in 12 cases and questionable in 14 cases. There was no histologic evidence of lymphoma in the remaining 12 cases. Immunohistochemistry performed on BM frozen sections demonstrated the T-cell origin of the infiltrating lymphoid cells in 24 of the 26 patients with unequivocal or questionable involvement. The malignant nature of these cells was suggested by demonstration of an aberrant T-cell phenotype identical to that observed in the other sites of involvement. In addition, in four of the 12 cases with apparently normal BM at routine light microscopy, immunohistochemistry revealed a minimal but phenotypically abnormal T-cell population, suggesting mild infiltration by lymphoma. These combined histologic and immunohistochemical data documented a high incidence (73%) of BM involvement by PTCL. In addition, a very peculiar sinusal pattern of BM involvement was found in five patients who presented an unusual type of hepatosplenic T-cell lymphoma expressing the gamma delta T-cell receptor. The present study demonstrates the high incidence of BM involvement by PTCL and emphasizes the value of frozen section immunohistochemistry to establish this diagnosis, especially when routine light microscopy findings are questionable.