SBP is an infection of ascites that occurs in the absence of a local infectious source. It is mainly a complication of cirrhotic ascites, with a prevalence of 15% to 19% (when culture-negative cases are included). Gram-negative enteric bacteria are the causative agents in more than 70% of cases. SBP is probably the consequence of bacteremia due to defects in the hepatic reticuloendothelial system and in the peripheral destruction of bacteria by neutrophils, with secondary seeding of an ascitic fluid deficient in antibacterial activity. Patients with advanced liver disease and low ascitic fluid protein concentrations seem to have an increased susceptibility to SBP. A diagnostic paracentesis should be performed in any cirrhotic patient who suddenly deteriorates or presents with any compatible symptom of SBP, most frequently fever or abdominal pain, or both. A PMN count greater than 500/mm3 is indicative of SBP, and treatment with intravenous broad-spectrum antibiotics should be initiated immediately. Although the mortality of an acute episode of SBP decreases with early therapy, it is still high (approximately 50%), and patients who survive an episode of SBP have a high frequency of recurrence. Mortality seems to be related to the severity of the underlying liver disease, because only a third of patients die from sepsis and prophylactic antibiotics decrease the frequency of SBP but do not seem to improve long-term survival.