Background: An aneurysmal bone cyst (ABC) is a benign lesion occurring mainly in children. Generally it is found in the metaphyses of long bones and in flat bones. Osteolysis, bone expansion, thinning of the cortex, osseous septa and fluid levels belong to the typical imaging findings in ABC cases. The purpose of the study was to determine the value of imaging methods: plain x-ray films, US, CT and MRI for the diagnosis of aneurysmal bone cyst and to work out the diagnostic algorithm for an ABC suspected on plain radiograms. Material/Methods: The material consists of 72 patients (44 boys, 28 girls) aged 2-23 years (mean age 12 yrs) with bone lesions diagnosed as aneurysmal bone cysts on pathologic examination. In all patients, plain radiographs were performed, in 26 - sonography (US), in 41 - computed tomography (CT), in 8 - magnetic resonance imaging (MRI). All the examinations were analyzed according to own system of evaluation of typical presentation in the particular methods. The histopathological diagnosis was established after biopsy or surgery. Results: The aneurysmal bone cysts were most frequently located in the long bones of the lower limbs (43%), in the long bones of the upper limbs (26.4%) and in flat bones (25%). The localization was mainly in the bone methaphyses (56.6%). On pathological examination, primary ABC was diagnosed in 65.3% of cases, secondary ABC in 23.6%, ABC and concomitant osteitis in 7%, ABC and giant cell tumor in 2.7%, ABC and chondrosarcoma in 1.4%. The application of four methods resulted in 72.2% consistency between diagnostic imaging and pathological examination. In case of only one method used the consistency was 75% for MRI, 63% for CT, 55.4% for plain films and 50% for US. In 100% of misdiagnosed cases there were no septa within osteolytic lesions on plain films, whereas in 43.8% features of malignancy were found. In 53% of misdiagnosed cases no fluid levels were observed on CT and in 69% on US. Conclusions: 1. The diagnosis of aneurismal bone cyst based on plain radiography is possible in cases with typical radiological signs. 2. Application of other methods significantly increases the percentage of correct diagnoses. 3. Lack of fluid levels does not exclude the diagnosis of ABC. 4. Imaging features of malignancy do not exclude the diagnosis of ABC
