Many studies have been done in the later course of Sheehan's syndrome, but very few have documented the acute phase with clinical, endocrine and imaging data. We present the case of a young woman complaining of severe headache after delivery, who developed hypopituitarism. Magnetic resonance imaging (MRI) disclosed the presence of an enlarged non haemorrhagic pituitary gland. Follow-up MRI showed a spontaneous and rapid shrinkage of the pituitary, within 20 days, which appeared as an empty sella 3 months later. Sheehan's syndrome may initially closely mimic hypophysitis, or the necrosis of an adenoma. We discuss the differential diagnoses, important for the best therapeutic management.