Acute immobilization is associated with rapid loss of bone. Prevailing opinion, based on population cross-sectional data, assumes that bone mass stabilizes thereafter. In order to address whole-body and regional skeletal mass in long-term immobilization, monozygotic twins were studied, one of each twin pair having chronic spinal cord injury (SCI) of a duration ranging from 3 to 26 years. The research design consisted of the co-twin control method using 8 pairs of identical male twins (mean +/- SD age, 40 +/- 10 years; range 25-58 years), one of each set with SCI. The twins were compared by paired t-tests for total and regional bone mineral content (BMC) and bone mineral density (BMD) measured by dual-energy X-ray absorptiometry. Linear regression analyses were performed to determine the associations of age or duration of injury with the differences between twin pairs for total and regional skeletal bone values. In the SCI twins, total-body BMC was significantly reduced (22% +/- 9%, p<0.001), with the predominant sites of reduction for BMC and BMD being the legs (42% +/- 14% 35% +/- 10%, p<0.0001), and pelvis (50% +/- 10% and 29% +/- 9%, p<0.0001). Duration of SCI, not age, was found to be linearly related to the degree of leg bone loss in SCI twins (BMC: r(2) = 0.60, p<0.05; BMD: r(2) = 0.70, p<0.01). Our findings suggest that pelvic and leg bone mass continues to decline throughout the chronic phase of immobilization in the individual with SCI, and this bone loss appears to be independent of age.