Invasive aspergillosis has become a major cause of infection-related mortality in nonneutropenic patients after allogeneic stem cell transplantation (SCT). To assess the potential role ofAspergillus-specific T-cell responses for the successful control of invasive aspergillosis, lymphoproliferative responses toAspergillus fumigatus antigens were studied in healthy individuals, patients with evidence of invasive aspergillosis, and patients late after allogeneic SCT. In healthy individuals, a positive lymphoproliferative response was documented to cellular extracts of A fumigatus (14 of 16), the 88-kDa dipeptidylpeptidase (4 of 16), and the 90-kDa catalase (8 of 11). A predominant release of interferon γ (IFN-γ) in culture supernatants on stimulation with A fumigatus antigens was demonstrated in 13 of 17 healthy individuals, indicating a T_H1 response. In patients with clinical evidence of invasive aspergillosis, a favorable response to antifungal therapy was found to correlate with a higher IFN-γ/interleukin 10 (IL-10) ratio in culture supernatants (n = 7; median ratio, IFN-γ/IL-10 = 1.0; range, 0.09-24.8) compared to 10 patients with progressive or stable disease (median ratio, IFN-γ/IL-10 = 0.1; range, 0.002-2.1; P = .04). Steroid treatment was found to suppressAspergillus-specific lymphoproliferation (P = .037) and release of IFN-γ in culture supernatants (P = .017). In contrast to cytomegalovirus- and tetanus toxoid–specific T-cell responses, Aspergillus-specific T-cell reconstitution late after allogeneic SCT was characterized by low stimulation indices and a low IFN-γ/IL-10 ratio. In addition, phosphoantigen-reactive Vγ9/Vδ2 T-cell clones from healthy individuals were found to produce significant amounts of tumor necrosis factor in response to A fumigatus antigens. In conclusion, these results further support the hypothesis that T cells contribute to the host defense against A fumigatus.