Summary: Case histories are presented of 13 women admitted to hospital with acute abdominal pain occurring between the twelfth and thirty‐eighth weeks of pregnancy. In the 8 patients in whom laparotomy was performed a mild peritonitis, without obvious cause, was found; virus was cultured from each of these patients — from peritoneal fluid in 3 and from the faeces in 10. The antigenically related echoviruses types 1 and 8 were prominent amongst the strains isolated. Controls, pregnant women without acute abdominal pain, though not well matched in time, showed a very much lower incidence of viral infection. it is suggested that these 13 cases are examples of mild peritonitis in pregnancy caused by viral infection.