History and clinical findings of 18 cases of adenosquamous carcinoma of the skin appendages found among 135 cases of primary carcinoma of the vulva seen at the University of Minnesota Hospital between 1951 and 1970 were analyzed. In addition, two recent cases of this tumor were studied with conventional transmission electron microscopy. Adenosquamous carcinoma of the vulva showed poorer survival and a higher rate of lymph node metastases than squamous cell carcinoma of the corresponding stages (carcinoma in situ excluded). In four out of thirteen cases, the metastatic lesions in the lymph nodes retained glandular pattern. The ultrastructure showed mucin-producing columnar cells lining glandular lumina, and poorly differentiated squamous cells elsewhere; further, cells of the intermediate type between the two were present. This study indicates that adenosquamous carcinoma of the vulva is a distinctively separate entity from squamous cell carcinoma of the vulva, and possibly arises from mucin-producing cells of the skin appendages as suggested by Johnson and Helwig.