This report concerns the first cases of Vibrio cholerae infection reported in the United States since 1911. The infections were acquired accidentally by two laboratory technicians who had not been immunized previously with cholera vaccine. Mild clinical symptoms were observed in the first patient to become ill but in the second patient, the clinical symptoms were fulminating. Little microscopic evidence of an acute enteritis was found on examination of specimens of small-bowel mucosa taken from these two patients during the acute phase of their V cholerae infections but intravillous capillary dilatation was marked so that grossly the bowel mucosa was hyperemic in appearance. An in vivo change was observed in the antigenicity of the infecting strain of V cholerae (Inaba to Ogawa) during the course of the infections.