This comprehensive case review of hantavirus pulmonary syndrome (HPS) during pregnancy in 5 women characterizes the effect of Sin Nombre virus infection on maternal and fetal outcomes. Histopathologic, serological, and clinical information were evaluated for evidence of vertical transmission. Maternal ages ranged from 20 to 34 years and gestational ages from 13 to 29 weeks. Symptoms, physical findings, and laboratory values other than those related to pregnancy were not noticeably different from those of nonpregnant patients with HPS, although fevers were somewhat lower. One maternal death and 2 fetal losses occurred. Gross, microscopic, and immunohistochemical examination for hantavirus antigen were done on 2 fetal autopsies and 3 placentas showing no evidence of transplacental hantavirus transmission. There was no serological evidence of conversion in the 3 surviving children. Maternal and fetal outcomes of HPS appear similar to those of nonpregnant HPS patients and of pregnant patients with other causes of acute respiratory distress syndrome. No evidence of vertical transmission of Sin Nombre virus was found.