Using the open fontanelle as an acoustic window brain tumours were diagnosed by gray scale ultrasonography in 3 infants aged 1 day to 5 months. The tumours were characterized by their echo dense structure and their good delimination from the surrounding brain. In 2 children the tumour was localized infratentorially (medulloblastoma and unclassified neuroectodermal tumour) and had caused an occlusive hydrocephalus. Both children died aged 3 and 5 months in central nervous dysregulation. One child suffered from plexus papilloma which had caused a hypersecretory hydrocephalus. After resection of the tumour the hydrocephalus decreased without any further treatment. Comparison with axial computed tomography and autopsy findings showed, that gray scale ultrasonography is equally efficient in diagnosing brain tumours and associated hydrocephalus.