Parainfluenza virus (PIV) infections can cause serious respiratory infections and death in immunocompromised patients. No antiviral agents have proven efficacy against PIV, and therapy generally consists of supportive care. DAS181, a novel sialidase fusion protein that temporarily disables airway epithelial PIV receptors by enzymatic removal of sialic acid moieties, has been shown to inhibit infection with PIV strains in vitro and in an animal model. We describe here the clinical course of 2 immunocompromised patients with PIV-3 infection, one with a history of lung transplantation and the other neutropenic after autologous hematopoietic stem cell transplantation for multiple myeloma. Both patients had substantial clinical improvement in respiratory and systemic symptoms after a 5-day DAS181 treatment course, although the clinical improvement in the autologous stem cell transplantation patient also paralleled neutrophil engraftment.