BACKGROUND: Lymphocytes seen during the chemotherapy of childhood ALL are not fully understood regarding their clinical significance. The lymphoid aggregates found during the complete remission period are more confusing. We investigated the characteristics of lymphoid aggregates and the clinial course of children with these in the marrow during the chemotherapy of childhood ALL. This is the first study about this subject. METHODS: From January 1996 to April 1998, 210 bone marrow specimens were diagnosed as complete remission status of ALL and among them, ten patients (4.8%) showed lymphoid aggreagates on the marrow clot sections at the time of complete remission. We reviewed bone marrow specimens, performed immunohistochemical stains for CD3, CD 10 and CD79a and investigated the clinical course. RESULTS: The ten cases were composed of nine ALL, L1 and one ALL, L2. All of them were treated under guidance of the CCG (children's cancer group) protocol. Fourteen lymphoid aggregates from ten cases were found. They showed mean number of 1.4 per clot section, mean diameter of 132 micrometer, regular (36%) or irregular (64%) margin and composition of mature lymphocytes (21%), immature lymphocytes (29%) or mixed pattern (50%). The mean interval between the diagnosis and the emergence of lymphoid agregates was 29 months (2~55 months). One patient in the course of consolidation chemotherapy expired due to upper gastrointestinal bleeding and other nine cases are still in the continuous complete remission state. The lymphoid cells consisting of lymphoid aggregates showed positive reaction only for CD79a and negative reactions for CD3 and CD10. CONCLUSION: Lymphpoid aggregates found at the time of complete remission are collections of regenerating B-lymphocytes and they are not residual leukemic blasts, and show no effect on the complete remission state.