The purpose of this study is to define the clinical features and characteristics of radial tears in the root of the posterior horn of the medial meniscus and to report the outcome of arthroscopic treatment. Arthroscopic meniscus surgery was performed on 7,148 knees. Of those, 722 (10.1%) were radial tear in the root of the posterior horn of the medial meniscus. We reviewed the medical records from a random sample of 67 subjects studied (mean age 55.8 years, range 38-72, mean follow-up period 56.7 months, range, 8-123), which included surgical notes and detailed arthroscopic photographs of 70 knees. All patients were treated with arthroscopic partial meniscectomy. The age distribution, preoperative physical signs, results of magnetic resonance imaging , body mass index, and surgical findings of the study subjects were analyzed and the clinical results were graded with the Lysholm knee scoring scale and a questionnaire. Radiologic evaluation consisted of preoperative and at the latest follow-up radiographs. Eighty percent of the patients were older than 50 years, and 80.6% were either obese or morbidly obese. The mean Lysholm score improved from a preoperative value of 53 to a value of 67. The average preoperative Kellgren-Lawrence radiograph grade was 2 (range 0-3 points), a value that increased to 3 (range 2-4) at the latest follow-up, which showed a significant worsening. The preoperative MRI was reevaluated after the arthroscopic confirmation of a medial meniscal root tear. A tear could be demonstrated in only 72.9% of the patients, the rest of whom demonstrated degeneration and/or fluid accumulation at the posterior horn without a visible meniscal tear. Radial tears in the root of the medial meniscal posterior horn, which may not be visible in about one-third of the preoperative MRI scans, are common. That type of meniscal tear is strongly associated with obesity and older age and is morphologically different from the degenerative tears that often occur in the posterior horn. Partial meniscectomy provides symptomatic relief in most cases but does not arrest the progression of radiographically revealed osteoarthritis.