A case is reported of severe osteopenia caused by heparin treatment of thrombosis in the eleventh week of pregnancy followed by heparin prophylaxis (5000 IU three times daily) during pregnancy and lactation. The mother complained of back pain during the last two weeks of pregnancy. Six weeks post partum, generalized osteopenia in the skeleton was diagnosed and a compression fracture of the body of the sixth thoracic vertebra. During pregnancy the mother had relatively low serum concentrations of 1,25(OH)2D, the active metabolite of vitamin D, and six weeks after delivery the serum concentration had fallen to about 50% of the lowest reference level. Eight and fourteen weeks after delivery, when heparin treatment had been discontinued, the serum concentrations of 1,25(OH)2D were within the reference range for non-pregnant adults.