We describe 13 cases of a peculiar lymphoid tumour containing very large numbers of reactive histiocytes. The tumours occurred in young patients (mean age 14.8 y) who presented with systemic symptoms and superficial lymphadenopathy. Microscopic examination revealed a diffuse effacement of lymph node structure due to the presence of histiocytes intermingled with a variable number of anaplastic large lymphoid cells. The latter, in some cases, were isolated, while in others they were arranged in clusters or were diffusely present in residual sinuses. The large anaplastic cells expressed the activation markers CD30 (Ki-1), CD25 (interleukin-2 receptor), CD70 (Ki-24) and Ki-27, as well as varying combinations of T-associated molecules. The histiocytes expressed lysozyme and the CD11b (C3bi-R), CD11c (p150, 95) CD14, CD68 (KPI) and Ber-Mac3 antigens. Double staining with the antibody Ki-67 demonstrated that the proliferating components were the CD30-positive cells and not the histiocytes. T-cell receptor beta gene rearrangements were shown in three cases tested. The patients responded well to aggressive chemotherapy and nine are still alive, eight in complete remission. It is suggested that the tumour represents a well-defined clinico-pathological entity originating from activated T-lymphocytes.