The grey scale ultrasound features of 30 cases of Wilms' tumour examined using a water delay scanner are presented. Eighty-seven percent of masses were of uniform texture, with echogenicity equal to or slightly greater than that of liver with small hypo-echoic areas. There were no totally cystic tumours in the series. In 21 cases the mass was clearly arising from the ipsilateral kidney, but in 9 cases the ipsilateral kidney could not be distinguished from the tumour. The state of the intrahepatic inferior vena cava (IVC) was correctly diagnosed in 27 cases and further evaluation was needed in two patients in whom this segment could not be demonstrated. If this segment is sonographically normal, then the IVC need not be evaluated further. Differential diagnosis is discussed, with particular reference to neuroblastoma. In the majority of patients ultrasound is the only modality needed to diagnose Wilms' tumour pre-operatively.