Most of the adolescent girls complain of pain during periods. The incidence of dysmenorrhea is 25%. A 17 year old girl came with right sided pain which was mistaken for appendicitis and had laparoscopic appendicectomy. She was referred to gynecology outpatient department since the pain on the right side was not relieved even on medical treatment. Repeatedly she took leave from her school. At one stage patient was thought to be malingering and was referred for psychotherapy. MRI showed class IV bicornuate unicollis uterus, with partially underdeveloped rudimentary horn with functional endometrium. The provisional diagnosis prior to laparotomy was bicornuate uterus but on surgery it was found to be a unicornuate uterus with non- communicating rudimentary horn on the right side with functioning Endometrium. There was no associated renal abnormality and endometriosis. The functioning unicornuate horn was attached to the uterus by a fibrous band which was excised easily. The patient was relieved of dysmenorrhea. In unilateral dysmenorrhea the probability of congenital malformation should be thought of. Early diagnosis and management will help to prevent a lot of complications like endometriosis, infertility, ectopic pregnancy and rupture of underdeveloped pregnant horn.