A previously unrecognized disease, hantavirus pulmonary syndrome, was described following an outbreak of severe, often lethal, pulmonary illness in the southwestern United States in May-June, 1993. We have now studied the morphologic features of the causative agent, Sin Nomber virus (SNV), by thin section electron microscopy and immunoelectron microscopy of infected Vero E6 cells. SNV virions were roughly spherical and had a mean diameter of 112 nm. They had a rather dense envelope and closely apposed fine surface projections, 7 nm in length. Filamentous nucleocapsids were present within virions. Viral inclusion bodies were present in the cytoplasm of infected cells; these appeared granular or filamentous, depending on the plane of section. All of these characteristics were similar to published descriptions of other hantaviruses; however, unlike all other hantaviruses and virtually all other member viruses of the family Bunyaviridae which bud upon smooth intracytoplasmic membranes, SNV budding occurred almost entirely upon the plasma membrane of infected cells. Virus budding was associated with the formation of long 28 nm diameter tubular projections. Occasional elongated 47 nm diameter virus-like particles were seen to bud upon intracytoplasmic membranes. As shown by immunoelectron microscopy, viral antigens were localized over virions, inclusions, and tubular projections associated with virion morphogenesis.