Thrombocytopenia is commonly found in patients with serious infection. To investigate this phenomenon, 14 consecutive patients (68 ± 10 years) who underwent laparotomy for bowel perforation and culture-proven peritonitis were prospectively studied. Ten noninfected laparotomy patients served as a control group. None of the 10 control patients developed thrombocytopenia. Of the infected group, 12 of 14 patients (85%) developed thrombocytopenia (< 100,000/mm3). One patient (9%) developed disseminated intravascular coagulation (DIC). Of the remaining 11 patients with thrombocytopenia, platelet counts fell from preoperative level of 350,000 ± 166,000 to 54,000 ± 30,000 (p < 0.001) and reached this nadir 4.3 ± 2 days after surgery. There was no statistically significant difference in prothrombin time, partial thromboplastin time, or fibrinogen levels before versus after operation in this group. Bleeding times in seven patients were 5.5 ± 2 minutes, and bone marrow examination in five patients with platelet count of < 50,000/mm3 revealed normal or increased megakaryocytes. No patient in this group bled, and medications held, or received platelet transfusions. Platelet counts increased > 100,000/mm3 at a mean of 8.9 ± 4.1 days after operation. It is concluded that (1) thrombocytopenia is common following surgery for intra-abdominal infection, (2) is not usually associated with DIC, clinical bleeding, or coagulation abnormalities, (3) does not commonly result from bone marrow suppression, and (4) is transient and does not require routine platelet transfusions.