Increased proportions of CD10, CD19 and HLA-DR+ cells in bone marrow specimens of children with nonmalignant hematologic conditions have been recently identified. In the last 3 years we have been following up 11 children with hematologic disorders who revealed an abnormal immunophenotypic pattern in their bone marrow specimen. The objective of this study was to evaluate this abnormality in association with the patients' clinical course and development of a B-lineage malignancy. The study population consisted of 11 children aged 4 mo to 13 yrs (mean 4,8 yrs) who had undergone bone marrow diagnostic procedures for haematological disorders: neutropenia (2), thrombocytopenia (8) and Fanconi anemia (1). Bone marrow morphological studies in smears and biopsy samples revealed increased numbers of immature cells(28-60%) in 2/11 patients. Flow cytometry analysis of bone marrow biopsy samples (XL-Coulter) showed in all patients an increased percentage of immature B - cell precursors which expressed the lymphocyte surface markers CD19, CD10 (CALLA) and HLA-DR. Results from the immunophenotyping analysis were expressed as percentage of positive cells. The mean values of CD19+, CD10 and HLA-DR+ cells were 46% (±18,5), 44,5%(±23) and 56,3% (±22) respectively. Patients were treated with the standard protocole treatment according to their diagnosis and were followed up for 6 mo - 3 yrs (mean 2 yrs). Clinical and laboratory evaluation during the follow up time show that 9/11 children have recovered completely and 2/11 are still on treatment for their cytopenia. Nevertheless, none of these children has developed malignant lymphoproliferative disease. These findings indicate that an abnormal immunophenotypic pattern is not always consistent with heamatologic malignancy. However, patients with all these findings must have a long-term follow up in order to reach a definite diagnosis.