Trabecular bone density at the distal radius and cortical bone density at the midradius were measured in four randomized groups of women before and after 4 months administration of AAACa, oyster shell heated under reduced pressure with addition of heated algal ingredient (HAI) (group A); AACa, the same preparation without HAI (group B); CaCO3 (group C); and placebo (group D) in a double-blind system using peripheral quantitative computed tomography (pQCT) with lumbar spine density measurement by dual-energy X-ray absorptiometry (DXA). Groups A, B, and C received 900 mg/day elemental calcium and D received none. In subjects of group A, but not B, C, and D, radial trabecular bone density increased significantly, to 106.2% +/- 2.1% of the initial value (mean +/- SEM). The increase of trabecular bone density was significantly different from the placebo group (D) only in AAACa (group A) and not in AACa (group B) and the calcium carbonate (group C). Cortical bone density increase was also greater in group A (but not in B and C) than in D. Lumbar spine density did not change significantly. AAACa was apparently more effective, increasing trabecular bone density more than AACa and CaCO3 containing the same amount of elemental calcium.