Thrombocytopenia usually has a moderate course in full-term babies. Here, however, we describe two newborns with serious complications due to neonatal alloimmune thrombocytopenia. One patient was transferred to the paediatrician because of pallor, a swelling on the head and petechiae. He had a subgaleal hemorrhage. Following a platelet transfusion he made a complete recovery. The other presented with thrombocytopenia and petechiae and was treated with intravenous immunoglobulin. Several days later the patient started vomiting. Cranial ultrasound showed hydrocephalus most probably arising from an intraventricular haemorrhage following the thrombocytopenia, for which he received a ventriculoperitoneal drain. After this he made a successful recovery. Although neonatal alloimmune thrombocytopenia is a rare condition it can have serious consequences for the newborn and for subsequent pregnancies. It is important that treatment be started early and that cranial ultrasound always be performed.