We present a case of an unusual cause (pyometrium) for an acute abdomen in a four-month-old infant. A 4-month-old baby presented late one evening to the emergency department having been unwell for the past 2 weeks with a gastroenteritis-type illness. Her diarrhoea and vomiting (which was non-bilious) had settled in the 24 h before her attendance. Her parents said that she had not been herself; her appetite had decreased and she had cried inconsolably just before her attendance. They had noticed that her abdomen was becoming increasingly swollen and this was their main reason for presenting to the emergency department. There was no history of fever. In the 24 h before attendance, she had not vomited but had passed four yellow-coloured stools. Her medical history showed that she was born at 37 weeks’ gestation and required admission to the special care baby unit due to …