Retroperitoneal bölgenin enfeksiyonları belirsiz sinsi semptomlar ve nonspesifik klinik bulgu ve semptomlar nedeniyle tanı konulması zordur. Retroperitoneal abse acil serviste nadiren karşılaşılan potansiyel olarak hayatı tehdit edici intra-abdominal enfeksiyondur. Etyolojik olarak primer ve sekonder olarak ikiye ayrılır. En yaygın nedenleri üriner sistem enfeksiyonları, gastrointestinal sisteme ait hastalıklar, vertebra ve böbrek tüberkülozudur. Daha az görülen nedenleri arasında kemik enfeksiyonları, travma, uzak enfeksiyonlardan hematojen yolla yayılım ve kanser hastalıklarını içerir. Biz burada 51 yaşında mental retardasyonlu, başka ek hastalığı bulunmayan, tanı konulamamış ve etiyolojisi belirlenemeyen dev bir retroperitonel abseli hastayı tartıştık. Sonuç olarak bu tür hastaların zamanında tanı ve tedavisi için yüksek şüphe endeksi gereklidir özellikle mental retardasyon da bulunuyorsa. 

Retroperitoneal abscess (RA) is an unusual but potentially life-threatening intra abdominal infections which is rarely encountered in emergency departments (ED) (1,2). Insidious clinical manifestations and occult nature of abscess make it diagnostic challenge and causes delays and missed diagnosis that leads to prolonged sepsis, and increased morbidity and mortality rates (3,4). Retroperitoneal abscess may be classified as primary if the infection results from hematogenous spread or secondary if they are related to an infection in an adjacent organ. In a small percent RA may be idiopathic (4,5) which infections may be monomicrobial but are in most cases polymicrobial (4). Most commonly origin of abscess is primarily urinary tract infection, followed in frequency bowel-related diseases such as diverticulitis (1,6), retroperitoneal appendicitis, pancreatitis, biliary, and peptic ulcer diseases spinal and renal tuberculous disease(3). But cases have been described resulting from bone infections, trauma, hematogenous spread and malignancies(2,4). The most commonly isolated pathogens are gram-negative bacilli such as Escherichia coli and Proteus mirabilis in frequency, but anaerobic species such as Bacteroides may also be found Grampositive cocci, mainly staphylococcal species and rarely streptococcal species, are usually isolated in cases of hematogenous spread(4). Manifest clinical symptoms include fever, abdominal and/or flank pain, lumbar mass, weakness, weight loss and anorexia (4). Mainly predisposing factors are diabetes mellitus and immunocompromised hosts (7). The most reliable and sensitive diagnosis tool remains Computed tomography CT scan (4,8). The treatment modalities consist of open surgery, percutaneous drainage and accompanied intravenous antibiotic administration (1,4). We reported here a case of RA secondary to unknown etiology in a patient with mentally retarded, whose specification was delayed for weeks before it diagnosed and reached huge size.