1. The skeletal effects of simple bed rest and re-ambulation were studied in a consecutive series of 34 patients (aged 18–60 years) hospitalized with low backache due to protrusion of a lumbar intervertebral disc. The bone mineral content of the second, third and fourth lumbar vertebrae was determined by dual-photon (153Gd) absorptiometry immediately after admission to the hospital, at the end of the bed-rest period (mean 27 days, range 11–61 days) and approximately 15 weeks later (range 11–24 weeks).
2. During recumbency a mean decrease in lumbar spine bone mineral content of 0.9% per week was observed.
3. Re-ambulation resulted in bone mineral gain, and restoration of lumbar spine bone mineral content was nearly complete after 4 months.
4. The findings suggest that the simple therapeutic bed-rest regimen leads to excessive vertebral bone loss. Recurrent bed-rest periods may predispose to spinal osteoporosis.