Immune suppression of hematopoiesis has been implicated in the pathogenesis of acquired aplastic anemia. Similarly, abnormalities of T cells and bone marrow stromal cells have been reported in aplastic anemia, as has abnormal cytokine production. Stem cell factor (SCF) (also known as kit ligand, mast cell growth factor and Steel factor) is an early acting hematopoietic growth factor that is produced by a variety of mesenchymal cells including bone marrow stromal cells. To determine whether abnormalities in the production of stem cell factor occur in aplastic anemia, we evaluated serum levels of SCF in 25 patients with aplastic anemia. The mean serum levels of SCF in aplastic anemia patients were significantly lower (2.7 +/- 1.1 ng/ml) than those found in a comparable population of 257 normal controls (3.3 +/- 1.1 ng/ml) (P = 0.011). The SCF level did not correlate with patient age, the duration of aplastic anemia or with white blood cell count, platelet count or hematocrit. Although there is no direct evidence that lower SCF serum levels contribute to the pancytopenia seen in this disorder, identification of underlying abnormalities that can result in the deficient production of stromally derived hematopoietic growth factors will be important.