A 38-year-old patient with systemic lupus erythematosus presented with pulmonary infiltrates and hypoxemia for several months following immunodepleting autologous hematopoietic stem cell transplantation. She was treated for influenza, which was isolated repeatedly from ororpharynx and bronchoalveolar lavage fluids, and later empirically for lupus pneumonitis, but expired 6 months after transplant. Autopsy findings failed to show influenza in the lungs or lupus pneumonitis. A novel generic PCR-based assay using degenerate primers identified human coronavirus HKU1 RNA in bronchoalveolar lavage fluid at autopsy. Coronavirus was confirmed by virus-specific PCRs of lung tissue at autopsy. Electron microscopy showed viral particles consistent with coronavirus HKU1 in lung tissue both at autopsy and from a previous biopsy. While human coronavirus HKU1 infection is not usually severe, in highly immunocompromised patients, it can be associated with fatal pneumonia.