The aim of this study was to describe and compare bone mineral density (BMD) development in Norwegian women and men aged 25-44 years in a population-based, longitudinal study. BMD was measured twice at distal and ultradistal forearm sites by single x-ray absorptiometry in 258 women and 147 men (mean follow-up time, 6.4 (standard deviation, 0.6) years). At the distal site, a small annual gain of approximately 0.1% became a small loss beginning at age 34 years in men and age 36 years in women. At the ultradistal site, BMD change was predicted by age in women only, and bone loss started at age 38 years. A high degree of tracking of BMD measurements was observed for both sexes and both sites, r > 0.93. Depending on total BMD change, participants were grouped into "losers", "nonlosers", and "gainers", and more than 6% lost more than the smallest detectable amount of BMD: > or =3.46% at the distal site and > or =5.14% at the ultradistal site. In both sexes, bone mineral content (grams) decreased, whereas area (centimeters squared) increased significantly in "losers" compared with "gainers". This finding might represent physiologic compensation preserving bone strength. No cohort effects were observed when 1994 and 2001 measures from similar age groups were compared.