Common side effects of şuoxetine are headache, irritability, sleep abnormalities, nausea, diarrhea, and weight loss. Fluoxetine-induced menstrual cycle changes are not common situations. In this article, emergence of oligomenorrhea with the use of şuoxetine in an adolescent patient will be presented. The case was discussed in the light of literature. A 17-year-old female patient was admitted to our clinic with complaints of deprivation of enjoyment, reluctance, poor appetite, frequent headaches, nausea, insomnia and decline in school performance. The patient was diagnosed to have Major Depressive Disorder (MDD) according to DSM-IV-TR criteria after a complete clinic assessment and psychometric examination. Sertraline treatment with a dose of 50 mg/day was started. At the end of the 4th week of the medical treatment, because the patient’s complaints did not improve, sertraline was replaced by şuoxetine 20 mg/d. Routine clinic examinations were performed twice a month and after the 1st month of treatment, significant improvements were achieved in depressive complaints. However, the patient reported a prolonged menstruation period, which occurred once within two months after she had started taking şuoxetine. She had reported no such side effect that were observed with sertraline before. Besides, she had stated that her menstruations were regulated since she was 14 years old and she had no accompanying disease, which could cause menstrual irregularity. Complete blood count, thyroid function tests, prolactin level, blood glucose, renal and hepatic tests were performed. All laboratory values were normal except the parameters addressing iron deficiency anemia (HGB: 11.9 gr/dL). Polycystic ovary syndrome was excluded using laboratory and ultrasonography analyses. In the literature oligomenorrhea is not reported to be a frequent side effect of şuoxetine treatment. In a study about the possible effects of şuoxetine on menstrual cycle, it has been reported that şuoxetine could change the menstrual cycle. The most frequently observed menstrual irregularity related to şuoxetine has been reported to be oligomenorrhea as was in our case. In the literature, most of the cases, which were related to şuoxetine-induced oligomenore are about premenstrual syndrome (PMS). In a study that aimed to explore the relationship between PMS and menstrual cycle disorder, it has been reported that women, who have irregular menstrual periods are found to have high incidence of PMS. To our knowledge, there is no reported case in the literature with such a side effect after şuoxetine treatment of MDD in adolescents. Menstrual irregularities frequently cause some problems on a variety of areas such as working performance, productivity, mood, memory skills and sleep. Knowing that the selective serotonin reuptake inhibitors are commonly utilized for depression in adolescence, it is important for clinicians to pay attention on possible menstrual cycle irregularities that would cause a variety of problems.