Ultrasound scanning has been used for some years in cancer therapy not only for the detection and staging of tumours, but also to aid radiotherapy planning. The modern ultrasound scanner, provided it is fully digital, is ideally suited to accurately obtaining certain cross sectional anatomical information. By running radiotherapy planning software on the ultrasound microcomputer, interfacing problems between scanner and planning computer have been avoided. This approach offers high precision with a modest capital outlay that should prove attractive to the smaller radiotherapy centres.