Two cases of Ki-1 lymphomas in childhood were analyzed immunohistochemically and immunoelectron microscopically. They expressed Hodgkin's disease associated antigen, Ki-1, interleukin-2 receptor (IL2R), OKT9, and HLA-DR. Histologically, the tumour cells were large in size with abundant cytoplasm and atypical nuclei. Lymph node involvement was characterized by parafollicular and marginal sinus infiltration. These features were identical to those reported in Ki-1 lymphomas. Electron microscopically tumour cells had abundant cytoplasmic organelles with pleomorphic nuclei but had no specific granules. Some tumour cells had marked interdigitation of cell membrane. Immunoelectron microscopically Ki-1 was positive on cell membrane. Tumour cells had no T-cell or B-cell antigens except for Leu-3 (T4). Unexpectedly they expressed epithelial membrane antigen (EMA) strongly. EMA was positive on cell membrane and in the cytoplasm. EMA was detected effectively in paraffin-embedded sections. Among the malignant lymphomas in childhood tested, two cases were EMA-positive. The pattern of EMA-reactivity and the histology were very similar to Ki-1 lymphomas. These results strongly suggest that Ki-1 lymphomas in childhood may arise from non-lymphoid haematopoietic cells and that EMA can be used as a new marker to distinguish certain type of Ki-1 lymphomas in childhood.