We describe a case in which a patient immunocompromised by acute myeloid leukemia experienced acute fulminating invasive fungal sinusitis followed by pneumonia and then death. Though the microbiology of the pneumonia could not be directly investigated, nasal lesions revealed fungal mycelium. Valsa sordida was consistently cultured from a biopsied sample. The fungus was identified to the genus level based on morphology in culture and DNA sequence homology, and then was placed at species level by means of phylogenetic analysis of the nuclear ribosomal internal transcribed spacer region. The fungus is in the order Diaporthales, family Valsaceae in the Ascomycota and is distributed worldwide as a pathogen of trees in the genera Populus and Salix. Koch's postulates were demonstrated to apply in a neutropenic rat model. The fungus was susceptible to antifungals with MIC-0 scores of 0.0313 mu g/ml for amphotericin B, 0.25 mu g/ml for voriconazole, 0.0313 mu g/ml for caspofungin, and MIC-2 of 16 mu g/ml for fluconazole. This is the first substantiated report of an isolate in the genus Valsa (anamorph Cytospora) being identified in human disease.