We have studied the bone mineral density (BMD) of the lumbar spine in two healthy population groups in Spain with similar characteristics but with different socio-economic levels. BMD was measured in 1116 individuals of both sexes in two selected groups: (A) 832 volunteers in the urban Barcelona area and (B) 284 volunteers from a suburb with lower socio-economic level. Individuals of group A have greater spine BMD than group B. The maximum difference was found in the group between 20 to 39 years: 5% (P less than 0.001) in men and 3% (P less than 0.05) in women. The patterns of bone loss in both groups were similar in onset, rate and quantity, suggesting a possible developmental cause for this difference. Bone loss in women began before the menopause and increased considerably in the following years. The BMD values show that most people at advanced age from the low socio-economic group cross the fracture threshold earlier than the first group.