Fourteen children with a primary myelodysplastic syndrome (MDS) were seen at this center over a 10-year period. Six of the patients, including two pairs of siblings, had a monosomy 7 population in their bone marrow. Seven patients had the clinical and laboratory features of "juvenile chronic myeloid leukemia." Three patients could be considered to have either the monosomy 7 syndrome or "juvenile chronic myeloid leukemia," indicating that these two entities are not mutually exclusive. All patients fulfilled the French-American-British (FAB) criteria for a myelodysplastic syndrome. Clonal chromosomal abnormalities were detected in 13 of the 14 patients, and consistently involved either monosomy 7, multiple abnormalities, and/or multiple clones. Hematopoietic progenitor assays of blood and marrow samples obtained from most patients showed abnormal progenitor frequencies, or differentiation patterns in culture (or both), often affecting the erythroid as well as the granulopoietic lineages. In particular, granulopoietic progenitors from four to six patients in the "juvenile chronic myeloid leukemia" category generated predominantly abnormal appearing macrophage colonies. Clinical outcomes were poor with rapid transformation to acute myeloid leukemia in most patients. All treated patients responded poorly to conventional chemotherapy, although in two cases remission was achieved with intensive therapy and allogeneic bone marrow transplantation. Childhood myelodysplasia includes a group of diseases that are clinically heterogeneous, and current terminology is confused and inconsistent. Until a better understanding of the biologic and molecular basis of these diseases is obtained, it is proposed that the use of the FAB categories developed for adult MDS might help to improve diagnostic precision and therapeutic comparisons.