Three patients with cerebellar infarcts and secondary obstructive hydrocephalus (hydrocephalus occlusus) are described, the condition being due to the space-occupying action of the ischaemia and oedema zone with compression and displacement of the fourth ventricle and/or obstruction of the aqueduct. Possible brain stem compression and danger of cerebellar tonsillar herniation were present. From the acute deterioration of the clinical picture, particularly of the state of consciousness, as well as from the findings obtained via computed tomography showing a widening of the inner ventricular space, it was imperative to perform emergency pressure-relieving drainage surgery with ventriculo-atrial shunts in all the cases described. The first two patients could be discharged postoperatively with successful treatment, only slight neurological disturbances remaining. This leads to the conclusion that a (malignant) cerebral infarct should be treated as an emergency case, in a somewhat similar way as in cases of cerebellar haemorrhage or other space-occupying lesions in the posterior fossa of the skull. To recognize the life-threatening exacerbation, sufficient observation and supervision will be necessary. Rapid deterioration in consciousness should be considered a sign of increasing intracranial pressure progressing with the development of hydrocephalus internus occlusus. After neuroradiological diagnosis, especially after verification via computed tomography, this should be interpreted as an indication for immediate neurosurgical intervention. This is the only way to keep the mortality rate satisfactorily low. Dichotomy of cerebellar infarcts into a benign type with spontaneous decrease of symptoms and signs, and a malignant type with development of hydrocephalus internus and increased intracranial pressure, such as in the cases described here, is suggested.