Marrow samples from 89 patients with aplastic anemia (AA) were evaluated for their ability to grow stromal layers in standard long-term marrow cultures (LTMCs). Results were highly variable: 6.8% failed to grow any stromal cells (group I); 42.5% either failed to grow to confluency or appeared to have a decreased number of adipocytes and/or macrophages (group II); and 52.8% appeared as normal confluent cultures with fibroblasts, adipocytes, and macrophages (group III). Analyses of patient data suggested that group I patients had a longer disease duration and poorer survival (P = .07). Enzyme-linked immunosorbent assay analysis of cytokine production was performed on 20 of the normal-appearing AA LTMCs and 12 LTMCs established from normal donors. Significant differences between the AA and control groups were apparent for macrophage inflammatory protein-1 alpha (MIP-1 alpha), interleukin-1 receptor antagonist (IL-1ra), granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), and leukemia-inhibitory factor (LIF). The most dramatic differences observed were elevated levels of MIP-1 alpha and GM-CSF and decreased levels of IL-1ra, particularly after IL-1 alpha stimulation. In contrast, IL-1 alpha stimulation of AA LTMCs produced levels of IL-6, LIF, and G-CSF comparable with those of controls. These data suggest that defects exist within the microenvironment of some AA marrows. Whether the majority of these defects are the cause or consequence of aplasia is not clear. However, we speculate that some of these abnormalities may contribute to the maintenance of the hypoplastic state and, in extreme cases, prevent engraftment of donor marrow.