Lateral spine dual-energy x-ray absorptiometry (DXA) selectively measures the trabecular-rich vertebral bodies without the contributions of the cortical-rich posterior elements of the spine and is less affected by spinal degenerative disease than posterior-anterior DXA. We tested whether lateral DXA detects vertebral osteoporosis more often and is more sensitive to age-related bone loss than posterior-anterior DXA in 193 healthy, community-dwelling men aged 51-81 years (mean +/- SD; 67 +/- 8 years). All men had supine lateral, posterior-anterior, and proximal femur DXA scans on a Hologic QDR 2000 densitometer. A subset (n = 102) had repeat scans after 4 years to determine annualized rates of change in bone mineral density (BMD). Age was inversely and significantly associated with BMD in the midlateral (r = -0.27) and lateral (r = -0.24) but not posterior-anterior (r = 0.04) projections. Midlateral (-1.43 +/- 3.48% per year; p = 0.0001), lateral (-0.27 +/- 1.68% per year; p = 0.12), and hip (-0.19 +/- 1.02% per year; p = 0.06) BMD decreased, whereas posterior-anterior BMD increased (0.73 +/- 1.11% per year; p = 0.0001) during follow-up. When compared with normal values in 43 men aged 21-42 years, mean T scores were significantly lower with lateral (-1.47 +/- 1.32) and midlateral (-1.57 +/- 1.36) than posterior-anterior (-0.12 +/- 1.30; p < 0.0001) DXA. Only 2.6% of the older men were considered osteoporotic (T score < or = -2.5) at the posterior-anterior spine, whereas 11.0% were osteoporotic at the femoral neck, 22.5% at the lateral spine, and 24.6% were osteoporotic at the midlateral spine. We conclude that supine lateral DXA identifies considerably more men as osteoporotic and is more sensitive to age-related bone loss than posterior-anterior DXA. Spinal osteoporosis may represent a substantially greater health problem among older men than previously recognized.