Smooth muscle tumors arising in the vulva are rare. Leiomyosarcoma is the most common variant of vulvar sarcoma, and very few cases have been reported during pregnancy. A 36-year-old woman presented with a progressively enlarging vulvar mass during pregnancy, diagnosed as a Bartholin's gland cyst. The lesion was resected at 38 weeks of gestation during cesarean section and diagnosis of myxoid leiomyosarcoma of the vulva was made. Six weeks later the patients were referred to our center and submitted to wide vulvar excision with groin lymph node dissection that revealed the presence of a small residual focus of leiomyosarcoma. At 30 months of follow-up the patient was well without any sign of recurrent disease. Leiomyosarcoma should be included in the differential diagnosis of vulvar masses; progressively enlarging vulvar lesion should be biopsied even during pregnancy. Leiomyosarcoma should be considered in the differential diagnosis of vulvar mass.