Although the positive association between physical activity and bone mineral density (BMD) is well established, few epidemiological studies have investigated the long-term associations between physical activity during adulthood and BMD later in life. The aim of this prospective, population-based study was to examine the association between leisure time physical activity in adulthood and areal BMD (aBMD) later in life. We examined 1,766 women and 1,451 men aged 20-54 years at baseline who were followed up 22 years later, as part of a population-based study in Norway. Leisure time physical activity was assessed by questionnaire at baseline and follow-up. aBMD was measured at the hip and forearm at follow-up, using X-ray absorptiometry. The association between aBMD and physical activity was analyzed using general linear models. We observed a positive linear trend in aBMD across physical activity levels in both women and men, after adjustments for baseline age, height, weight, and smoking status (P < 0.05). The relationship between aBMD and leisure time physical activity was consistent over different sites of the hip (total hip, femoral neck and trochanter area) and forearm (distal and ultradistal area). In a subsample of 2,436 men and women under 70 years of age, those who where sedentary at both baseline and follow-up (6%) had lower aBMD than those who were moderately active or active at both baseline and follow-up (71%) (P <or= 0.01). This study suggests that leisure time physical activity in adulthood is associated with higher aBMD and reduced risk of osteoporosis later in life.