The authors report three cases of diffuse aggressive non-Hodgkin's lymphomas (large cell, two; mixed small and large cell, one) arising in association with clinically silent small lymphocytic proliferations involving bone marrow, and lymph nodes. Immunologic marker studies suggested origin from a common B cell clone in two cases; in one case the large cell lymphoma lacked detectable surface markers. Some cases of diffuse aggressive lymphomas of adults may arise by large cell transformation of clinically silent small lymphocytic proliferations, a process akin to Richter's syndrome.