['The relative contributions of administered oestrogen and progestin to protection of the post-menopausal skeleton remain unclear. We have compared the effect on spinal bone mineral density of continuous combined oestrogen/medroxyprogesterone therapy with that of unopposed oestrogen in late post-menopausal women.', 'Observational study.', 'Seventy-three osteoporotic post-menopausal women attending the Auckland Hospital Bone Clinic. Twenty-three hysterectomized women were treated with unopposed oestrogen (0.625 mg oral conjugated oestrogens daily), and 50 non-hysterectomized women were treated with continuous combined oestrogen/medroxyprogesterone therapy (5 mg medroxyprogesterone acetate and 0.625 mg oral conjugated oestrogens daily).', 'Baseline and one-year measurements of lumbar spine bone mineral density were performed using dual-energy X-ray absorptiometry.', 'After one year of therapy, spinal bone mineral density increased by 6.6% (95% confidence interval 5.6-7.6, P < 0.001 vs baseline) in the combined oestrogen/medroxyprogesterone therapy group, compared to 4.0% (CI 2.4-5.7, P < 0.001 vs baseline) in the unopposed oestrogen group (difference between means 2.6% (CI 0.8-4.4, P < 0.01).'] Does medroxyprogesterone acetate enhance the spinal bone mineral density response to oestrogen in late post-menopausal women?