The third lumbar vertebrae of nine elderly subjects (average age, 81,4 $$ 6.7 years) graded osteoporotic and the second to fifth lumbar vertebrae of a 37-year-old man graded as normal were used to investigate microdamage accumulation during quasi-static compression loading with an acoustic emission detection system. Mechanical parameters (apparent elastic modulus, stress, and strain) and acoustic emission event count rates were measured simultaneously. The normalized mean value of any mechanical parameters of normal group was significantly high with respect to that of osteoporotic group. The normalized mean value of cumulative acoustic emission event counts to maximum stress of the normal vertebrae was substantially small with respect to that of the osteoporotic vertebrae (p < 0.0005, z-test). Postloading microradiographs displayed fracture lines adjacent to the and plates in six vertebrae of osteoporotic group. These results are consistent with the hypotheses that microdamages of osteoporotic vertebral bodies are generated and accumulate at lower strains than those of normal vertebras at a specific site.