The causes of peritonitis in childhood, particularly in infancy, were found to differ substantially from those in adult age. An overall account is given in this paper of general and differential diagnosis of extra-abdominal diseases which may be potentially mistaken for peritonitis. Clinical diagnosis is of crucial importance in childhood. Perforated appendicitis was the absolutely major cause of peritonitis in 119 children who received surgical treatment in the course of 18 years. An initial antibiotic combination of gentamicin with ampicillin, and metronidazole together with immune treatment have proved to be an effective therapeutic approach to this form of peritonitis.