The clinicopathologic findings of 13 patients having extramammary Paget's disease of the vulva are discussed with emphasis on its histogenesis and biological behavior. For the purpose of study and assessment of prognosis, these cases were divided into two groups, those with an underlying invasive cutaneous adnexal adenocarcinoma, and those lacking an underlying invasive lesion. Four cases contained invasive cutaneous adnexal adenocarcinoma; in one of these the invasion was superficial. Three of the cases with an invasive lesion and three other cases showed in situ adenocarcinoma of sweat glands. Surgical treatment is mandatory for both groups of patients. The prognosis was excellent for the patients having Paget's disease without an underlying invasive carcinoma. From the literature, the prognosis of those with an underlying invasive carcinoma of the vulva appears to be less favorable. Multiple surgical excisions may be required to control the recurrences and metastases. A frequent association with other internal malignancy was observed. In four cases, second malignancies were found. Of special interest was the demonstration in one case of columns of neoplastic cells extending from involved sweat glands to the surface epithelium via the intradermal sweat duct. Our study leads us to support the concept that the Paget's cells, in a number of cases, are derived from an underlying carcinoma in situ of sweat gland origin.