More than half of maternal deaths in the UK are due to pre-existing medical conditions, and medical emergencies often mimic acute obstetric conditions. An acute flare of systemic lupus erythaematosus, a thyroid storm or a phaeochromocytoma have many of the signs and symptoms of imminent eclampsia. Similarly, severe postpartum haemorrhage can result in diagnostic difficulties of medical conditions such as Addison's syndrome or acute renal failure. An acute collapse can be due to a pulmonary embolus, myocardial infarction, tachyarrhythmia or myasthenia gravis. These conditions are rare; however, unless they are considered in a differential diagnosis, they will not be diagnosed, to the detriment of the woman and her infant. This chapter deals with acute medical conditions occurring uncommonly in pregnant women. An obstetrician should know the initial steps to take in the emergency management of these cases.