Complete duplication of the uterus and cervix with a unilaterally imperforate vagina must be suspected when a unilateral pelvic mass terminates in a purpuric bulge of the lateral vaginal wall in a young woman with severely progressive dysmenorrhea. Intravenous pyelography will reveal renal agenesis ipsilateral to the imperforate vagina. The prompt and accurate diagnosis of this unusual anomaly should lead to transvaginal drainage of the retained menstrual fluids prior to irreversible damage of the pelvic viscera from chronic cryptomenorrhea.