The reliability of the clinical assessment, which comprised history, physical examination, and plain radiographs, was determined by comparing the initial preoperative diagnosis with the postoperative diagnosis as determined arthroscopically, and by comparing the results of the clinical evaluation with published reports of arthrography, computed tomography (CT), and magnetic resonance imaging (MRI). The study group included 206 patients (216 knees) scheduled for arthroscopic surgery for suspected internal derangements. The primary preoperative clinical diagnosis was correct in 175 knees (81%), with an overall accuracy of 93%, sensitivity of 89%, and specificity of 94%. The most common preoperative diagnoses were medial and lateral meniscal tears. Results of the clinical assessment were comparable or superior to published reports of arthrography, CT, and MRI. Thus, it may be unnecessary to routinely use these costly special studies to determine the need for arthroscopic surgery. We conclude that a thorough clinical assessment can provide sufficient information for the surgeon to make a definitive primary preoperative diagnosis, and that arthroscopy should not be performed without first completing a complete preoperative examination.