Patients with intra-abdominal processes that require prompt surgical intervention, including appendicitis, perforated viscus, ischemic bowel, volvulus, and bowel obstruction, often present with signs and symptoms of an acute abdomen. Several medical problems can mimic an acute abdomen. Overwhelming postsplenectomy infection is a life-threatening condition that can present with acute abdominal symptoms. The incidence of overwhelming postsplenectomy infection ranges from 1% to 25%, and is caused by Streptococcus pneumoniae in 50% of cases. Capnocytophaga canimorsus, a bacteria commonly found in dog saliva, accounts for less than 1% of cases. Overwhelming postsplenectomy infection has a rapidly deteriorating course that progresses to respiratory and renal failure, cardiovascular collapse, and death. The mortality associated with overwhelming postsplenectomy infection is 60% to 80%. Early diagnosis and institution of appropriate antibiotic therapy and supportive care is essential to improve patient outcome. A previously healthy woman who had undergone splenectomy secondary to trauma 11 years earlier presented with symptoms of an acute abdomen. A diagnosis of overwhelming postsplenectomy infection due to C canimorsus was made based on her peripheral blood smear and blood culture findings. Early aggressive care and antibiotic treatment resulted in a successful outcome for this patient with no long-term morbidity. This patient's clinical course demonstrates the importance of early diagnosis and treatment of overwhelming postsplenectomy infection.