Mucorales-specific T cells have been investigated in 28 hematologic patients during the course of their treatment. Three developed proven invasive mucormycosis (IM), 17 infections of known etiologies but other than IM, and 8 never showed fever upon the period of observation. The Mucorales-specific T cells may be detected only in patients with IM, at diagnosis and along the entire course of the IM, but neither before nor long time after the resolution of the infection. Such T cells produced predominantly interleukin-4, interferon-gamma (IFN-γ), interleukin-10, and to a lesser extent also interleukin-17, and belonged to either CD4+ or CD8+ subsets. The specific T cells producing IFN-γ were able to directly induce damage of Mucorales hyphae. None of the 25 patients without IM showed Mucorales-specific T cells. Specific T cells contribute to human immune responses against fungi of the order Mucorales and could be evaluated as a surrogate diagnostic marker of IM.