Thymic epithelial tumors consist of thymomas, thymic carcinoids, and thymic carcinomas. Most cases occur over the age of 20 years, and 70%of cases in patients over 40 years of age. Thymomas are extremely uncommon in children, accounting for less than 1%of childhood mediastinal tumours compared with 30%in adults and, because of their rarity may be difficult to diagnose. In this paper, we present a 12-year-old girl who presented with symptoms of pneumonia such as cough, fever and chest pain. Her chest x-ray showed expansion of the mediastinum and opacity in the right hemithorax. An anterior mediastinal mass was determined by chest computed tomography. Histological and immunochemicai examination of the surgically excised mass showed type B3 thymoma. The case is presented for its rarity in children. (Haseki Tip Bülteni 2013; 51: 200-02) Ozet Timik epitelyal tümörler; timomalar, timik karsinoidler ve timik karsinomalan kapsamaktadir. Hemen hemen tamami 20 yaj üstü olan olgulann %70'i 40 yaj üstüdür. Mediastinal tümörler içinde eriçkinlerdeki %30'luk oran ile karjilajtinldiginda çocuklarda %1 oranindaki görülme sikligi tani koymayi güciejtirir. Biz burada öksürük, atej ve gögüs agnsi gibi pnömoni bulgulan ile gelen 12 yasinda bir kiz çocugunu sunduk. Akciger grafisinde mediastinal genijieme ve sagda opasite görülen hastanin bilgisayarli tomografisinde kitle saptandi. Cerrahi olarak çikanlan kitlenin histolojik ve immünohistokimyasal incelemeleri B3 timoma oldugunu gösterdi. Olgu çocuklarda timomanin nadir görülmesi nedeni ile sunulmujtur. (The Medical Bulletin of Haseki 2013; 51: 200-02) Anahtar Kelimeler: Çocukluk çagi, mediastinal kitle, timoma