Bone mineral density (BMD) of total body, spine, and proximal femur and the percentage of body fat in 151 U.S.-born Japanese-American women and 137 Japan-born immigrant Japanese-American women living in San Diego, California were measured using dual-energy x-ray absorptiometry. These data were compared with unpublished data from Japanese women obtained in previous studies in Hamamatsu, Japan. The age-adjusted BMD for the spinal level, femoral neck, Ward's triangle, trochanter, and total body, respectively, of U.S.-born Japanese-American women were 10.2, 9.8, 9.9, 9.2, and 2.7% higher than those of native Japanese women. The U.S.-born Japanese-American women had significantly higher body fat than immigrant Japanese-American women. Furthermore, the immigrant women had higher BMD and higher body fat than their native Japanese counterparts; however, no significant total-body BMD differences were found among the three groups after age, height, and weight were adjusted. The U.S.-born Japanese-American women had BMD values equivalent to those of white normals at the spine and femur. Significant life-style differences between U.S.-born and immigrant Japanese-American women were noted. Weight, exercise, early menarche, and years of lifetime estrogen exposure correlated positively with BMD. The significant negative correlates of BMD were age, smoking, and percentage of body fat. Our study presents data suggesting that immigration to the United States has produced a higher BMD in Japanese-American women that is attributable to changes in life-style and diet.