Qualitative changes in the extracellular matrix have been reported in thickened ligamentum flavum (LF) in lumbar spinal canal stenosis (LSCS); however, no detailed quantitative reports are available to date. Degenerated and undegenerated LF were collected from patients who underwent decompression surgery for LSCS and lumbar disc herniation, respectively. Three cylindrical samples of 2.5 mm diameter were cut from one LF specimen, completely solubilized, and quantified glycosaminoglycan (GAG), collagen (COL), and elastin. The amounts of GAG, COL, and elastin in the LSCS group were significantly higher than those in the control group. Although the GAG was originally present only in trace amounts, GAG proportion, standardized by dry weight, was the highest increase rate. Besides component analysis, histologic analysis showed intraligamentous chondrometaplasia in the caudal portion of the degenerated LF. Increased GAG in LF may be a new therapeutic target in LSCS.