In a post mortem study of shock-induced changes in the bone marrow, marrow from six different parts of the skeletal system was examined in a total of 109 patients. The comparison of a group of 64 subjects deceased in shock conditions with a control group of 25 deceased without shock showed: In 76,6% of all patients with shock, fibrin networks were found in the sinus and perisinoidal interstice of bone marrow. This can be demonstrated abundantly in every fourth patient with shock. Microthrombi and emboli occluding vessels in the marrow were seen in only 6.3% of the shock cases (controls 0%). The different forms of fibrin precipitates appear most commonly after shock caused by infection. Platelet aggregates in the vessels are seen in 28.2% of all cases with shock (controls 8%). 5% of the cases with shock showed infarction-like necroses of the bone marrow and a further 23% necroses of small cell groups. In every fifth patient with shock, a great number of nucleated blood cells and their precursors were found in the marrow sinus (controls 8%). In a third group of 20 patients with serious illness alledgedly without shock, 45 per cent had a fibrin network when compared with the control group. When several of these findings are present simultaneously, one can apply the term "shock marrow".