In a prospective double blind clinical study of 70 patients with bony trauma and shock, 35 patients were allocated into each group receiving either a placebo or Aprotinin. In both groups there was a high incidence of pulmonary insufficiency and thrombocytopaenia. In the placebo group there was a higher incidence of pulmonary insufficiency, but not its severity, with greater volumes of blood transufsion and the most severe form occurred only after decompensated shock. Thrombocytopaenia showed a similar pattern in all patients from both groups falling to a mean minimum on the 2nd day and rising thereafter. In the placebo group counts were lower in those with decompensated shock, and the fall was more profound, rapid and earlier in patients developing severe pulmonary insufficiency. Counts of less than 100,000 on the first day were associated with a high probability of severe pulmonary insufficiency subsequently occurring and this finding was not associated with greater volumes of blood transfusion. In those patients receiving Aprotinin, the subsequent incidence of severity of pulmonary insufficiency was less and not associated with increased volumes of blood transfusion. The platelet counts were not lower in decompensated shock and there was a more rapid rise from the minimum level.