Fanconi's anemia (FA) is an autosomal recessive condition in which greater than 90% of the homozygotes develop aplastic anemia. To determine the relation between erythroid progenitors and clinical status, blood and marrow mononuclear cells were cultured in methyl cellulose with erythropoietin, plus other hematopoietic growth factors, and growth in normal oxygen (20%) was compared with growth in low, physiologic oxygen (5%). Peripheral blood cultures were performed from 24 patients, and marrows from six. Patients were classified into six clinical groups. Group 1: Severe aplasia, transfused; one patient; no erythroid progenitors. Group 2: Severe, transfused, androgen unresponsive; one patient; no blood burst-forming units-erythroid (BFU-E). Group 3: Androgen responsive; eight patients, with decreased blood BFU-E. Group 4: Aplastic, about to start treatment; two patients; below normal numbers of colony-forming units-erythroid (CFU-E) and BFU-E. Group 5: Stable, with mild anemia, and/or thrombocytopenia, and/or macrocytosis; seven patients; with below normal numbers of blood BFU-E. Group 6: Hematologically normal; five patients; blood BFU-E low normal to normal. One marrow had normal numbers of CFU-E and BFU-E. Incubation in 5% oxygen doubled CFU-E and BFU-E only in the patients with close to normal or normal growth in 20% oxygen. Hemin and interleukin-3 increased growth slightly in those cultures where there was some growth with erythropoietin alone. Our data show that there is a correlation between current clinical status and in vitro erythropoiesis. Cultures of erythroid progenitors may also be useful predictors of hematologic prognosis in FA, although our follow-up period is too short to prove this hypothesis.