To clarify the effect of mechanical stress on hypertrophy of the ligamentum flavum (LF), we compared the thicknesses, histological findings, and total collagen content of 161 lumbar LF in two groups defined by radiographic signs. LF with one or more signs of spondylolisthesis, disk degeneration, vacuum phenomenon, traction spur, or facet joint osteoarthrosis were considered degenerative; all others were considered nondegenerative. Thickness was measured by computed tomographic scan, and total collagen content by Woessner's method. We found that the ligament was thicker in the degenerative group (4.0 +/- 1.1 mm vs. 3.0 +/- 0.7 mm, p < 0.01), the degeneration of fibers more severe, and the total collagen content higher (51.9 +/- 14.5% vs. 40.3 +/- 3.9%, p < 0.05). Thus, mechanical stress is considered a cause of ligament hypertrophy.