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Notes: Summary The classification of acetabular fractures and especially the diagnosis of additional lesions can be misleading, when the personal experience is limited and the decisions are based only on conventional radiographs. The introduction of Spiral-CT with multiplanar reformations and 3-D views has improved the quality of visualization. Due to their higher costs, the need of these additional diagnostic tools is frequently questioned. This paper discusses the relevance of plain radiographs, 2-D-CTs, 3-D-CTs and Femursubtraction-CTs (FsCT) for the classification of acetabular fractures, based on a controlled study. Methods: Thirty physicians with different levels of experience in acetabular surgery were divided in three groups of 10 each: group I comprised residents without operative experience in acetabular surgery, group II was physicians with 3–10 years of operative experience, and group III was experts in acetabular surgery. A total of 10 complete radiographic cases of high quality providing all levels of preoperative diagnostics (plain radiographs, 2-D-CT, CT with multiplanar reformation, 3-D-CT, Fs-CT) of different acetabular fracture types were prepared. The task for each candidate was to classify the fracture according to Letournel and to identify all additional injuries within the hip joint (e. g. marginal impaction, head fractures, etc.). The different diagnostic “levels” could be ordered stepwise according to personal need and no time limit was given. The case was finished when the candidate presented his final diagnosis. The use of the different radiographs, the preliminary diagnosis, the changes in diagnosis, and the final decisions were recorded. These findings were correlated with the different levels of experience and against a “consensus classification” which was generated by thorough discussion, and the use of intraoperative information and postoperative radiographs not accessible to the candidates. Results: The “correct” fracture classification based on plain radiographs was: group I, 11 %; group II, 32 %; group III, 61 %. Based on 2-D-CT a “correct” diagnosis was reached by 30 % in group I, by 55 % in group II, and by 76 % in group III. With consideration of the “transient forms” in acetabular fractures based on Letournel and the 3-D-CT used mainly by group I, the rate of “correct” classifications rose to 65 % in group I, 64 % in group II and 83 % in group III. The modifiers were diagnosed “correctly” in group I by 37 %, in group II by 56 %, and in group III by 73 %. The use of the 3-D-CT and especially the Fs-CT by group I resulted in an improvement in the rate of correct classifications to 61 %, whereas in group II the Fs-Ct was used only exceptionally. The 2-D-CT was the basis for the diagnosis of the additional lesions in acetabular fractures within all groups resulting in 73 % complete diagnoses in group III. This study showed the importance of CT for the exact analysis and classification of acetabular fractures. In particular, the secondary reformations in CT and the 3-D-views dramatically improved the rate of “correct” classifications in the group of surgeons with limited personal experience in acetabular surgery. This allows the less experienced an acceptable level of “correct” diagnoses, so that the treatment options can be weighed correctly. Among the “experts” a rate of divergent classifcations of approximately 20 % was observed, especially in “transient” forms of acetabular fractures.