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Following a 7-month treatment with heparin, a young pregnant woman with a history of multiple phlebites complicated by pulmonary infarction developed osteoporosis with multiple compression fractures of the vertebrae. Histomorphometric analysis of a transiliac bone biopsy showed severe osteoporosis with rarefaction of the spongy bone, considerable increase of osteoclasts and fall in osteoblasts. The cortex was not atrophic and was the seat of active reconstruction. Clinical and experimental data have shown that prolonged, high dosage heparin treatment may result in such osteopenias. Heparin acts synergistically with the parathyroid hormone to stimulate bone resorption, at the same time increasing bone collagen metabolism and modifying its structure. Clinical heparin osteoporosis, however, is rare, which suggests that other factors intervene. The risk of osteoporosis must be taken into account when heparin therapy is considered in pregnant women who are at high risk of thromboembolism.