A 16 year-old boy of non-Hodgkin's lymphoma (NHL) was reported. Although Hodgkin's disease was suspected by the presence of Reed-Sternberg-like cells and lacunar cells histologically, a diagnosis of NHL was made because of atypism and monoclonality of the background's cells as well as the morphology of invasive cells in the bone marrow. The tumor cells expressed, CD2, CD3, CD4, CD5 and CD7 antigens, which corresponded to the phenotype of helper-inducer T-lymphocytes. In the analysis of their karyotypes, 16 out of 24 cells revealed normal karyotype, while all the rest showed near-triploidy. Common abnormality was identified as trisomies of No. 1, 3, 5, 16, 21 chromosomes, tetrasomies of No. 10, 19, 20 chromosomes, and 4q+, 7q+, 14p+. Multimodal chemotherapy was successful to induce the patient promptly into complete remission. He has been free from the disease for approximately 12 months. Thus far, triploid clones in hematopoietic malignancies have rarely been described. More importantly, the appearance of them in pediatric lymphoid neoplasms has not yet been reported.