To investigate the pathogenesis of hypertrophy of the ligamentum flavum, 45 cases of lumbar canal stenosis were evaluated by computed tomography scan and pathologic and immunohistochemical studies. The ligamentum flavum along with the medial one-third of the superior facet was obtained en bloc to include the enthesis. Statistically significant differences in transverse area and thickness of the ligamentum flavum were evident compared to the control group (P < 0.01). Pathogenesis of the hypertrophied ligamentum flavum was classified into three major groups: 1) fibrocartilage change due to proliferation of type II collagen, 2) ossification, and 3) calcium crystal deposition. It is stressed that marked proliferation of Type II collagen from the enthesis to the ligament side was revealed in the capsular portion of the hypertrophied ligament.