Late postinjury sepsis is largely the result of defective host defense including failure to maintain an adequate number of functioning phagocytic cells. In this study we used stem cell culture techniques to measure colony-stimulating activity and have quantitated the number of circulating myeloid stem cells to see if defects in granulopoiesis occur after major torso trauma. Forty-two acutely injured patients (13 blunt and 29 penetrating injuries; mean age, 29.7 years) undergoing laparotomy with an abdominal trauma index of 15 to 40 were studied prospectively. Blood samples were obtained on days 1, 5, and 10. Patients were segregated by injury severity: abdominal trauma index less than 25 (n = 25) versus abdominal trauma index greater than or equal to 25 (n = 17). The more severely injured (abdominal trauma index greater than or equal to 25) patients had fewer circulating granulocytes and monocytes. Colony-stimulating activity was below normal control levels in all patients and was decreased further with increased injury severity. The more severely injured patients had a blunted bone marrow response (significantly fewer circulating myeloid stem cells) and suffered more major septic complications (24% vs 8%). In conclusion, major trauma to the torso causes a paradoxic depression in granulopoiesis that worsens with increased injury severity and may contribute to late septic morbidity. This colony-stimulating activity deficiency state is similar to that seen after major burns and may be amenable to future modulation.