Cyclosporine (CyA) therapy has been shown to be effective in some patients with aplastic anemia. In an attempt to characterize aplastic patients likely to benefit from CyA therapy, we examined bone marrow mononuclear cells (BMMC) obtained before therapy from 23 patients with aplastic anemia, who were treated with CyA alone. Expression of four myelosuppressive cytokines, including tumor necrosis factor (TNF), lymphotoxin, macrophage inflammatory protein-1 alpha (MIP-1 alpha), and interferon-gamma (IFN-gamma) was examined using polymerase chain reaction (PCR)-assisted messenger RNA (mRNA) amplification. mRNA for TNF, lymphotoxin, and MIP-1 alpha was readily detectable at variable levels in BMMC from normal and transfused controls as well as in BMMC from aplastic patients. In contrast, IFN-gamma mRNA was only demonstrable in BMMC from some patients with aplastic anemia, irrespective of a history of transfusions. Of 13 patients who responded to CyA therapy and achieved transfusion-independence, IFN-gamma mRNA was detected in 12 patients, whereas the mRNA was only detectable in 3 of 10 patients refractory to CyA therapy (P = .003, Fisher's exact test). Follow-up examination of BMMC obtained from seven CyA-responding patients after hematologic remission showed disappearance of IFN-gamma mRNA in four patients. These results suggest that detection of IFN-gamma gene expression in pretreatment BMMC from aplastic patients using PCR may be helpful in predicting a good response to CyA therapy.