A computer system has been developed for the purpose of calculating patient treatment times and output calibration measurements and monitoring set-up conditions on a Philips RT100 superficial radiotherapy unit. With minor modifications it could be used with other superficial therapy units. The system has been developed around a Gemini Galaxy microcomputer wh ich has a Z80 processor and 64K of RAM. The software is written in BCPL, a highlevel structured language which runs under the CP/M operating system but is also available on the latest 32 bit IBM-compatible microcomputers running under the MS-DOS operating system. All programs are menudriven with facilities for returning to the start of the previous section by means of the “ESC” key. All data validated at the time of entry according to the data type and expected length or magnitude. A pROM-based system is used on the RT100 machine for calibration checks, treatment time calculations and set-up checks. This system has a safety feature in that contains two sets of identical data stored in two separate pROMs which are checked against each other before entry to the program is allowed. An additional safety feature is that the pROM data cannot be modified on this system by the system operator. The full system which includes the editor and hard-copy sections, runs on a microcomputer with Winchester or floppy disk and can be used in place of the pROM-based system if required.