This report describes the bone marrow findings in four patients whose marrow was involved by anaplastic large-cell Ki-1 lymphoma, an uncommon event in this type of lymphoma. In the marrow aspirate smears, the involvement was subtle, and was in the form of isolated large cells with irregular nuclear configuration, coarse chromatin, prominent nucleoli, and basophilic cytoplasm which might be vacuolated. One case showed paradoxically massive involvement in the trephine biopsy taken from the same site as the marrow aspirate. Reactive histiocytic proliferation with hemophagocytosis was also present. Since marrow aspirate or biopsy may be the first pathologic specimen examined in patients having anaplastic large-cell Ki-1 lymphoma, it is important to be able to recognize the small population of neoplastic cells, which should lead to prompt treatment or further investigations as deemed necessary.