About 18 months ago, a limited survey was made by the author, on behalf of the Scientific Sub-committee of the Hospital Physicists' Association, of the use of röntgen/rad conversion factors at some of the larger British and Commonwealth radiotherapy centres. The Sub-committee had hoped that some degree of uniformity in the use of these so-called f-factors might thus be achieved so that future comparisons of clinical results could be made on a consistent basis. Having examined the data, and considered the problem, the writer was unwillingly forced to the conclusion that, for the time being, exposure dose was likely to be a much more satisfactory basis for comparison, particularly for conventional X-ray qualities and for bone and its soft tissue components. The conclusions drawn from this survey were as follows. As far as soft tissues are concerned, with the possible exception of fat, acceptable conversion factors may be calculated to two significant figures for all energies of photon beams which are at present of clinical importance. The effect of energy degradation with depth of beam penetration in tissue and with field area appears to introduce an uncertainty of only 2 or 3 per cent. The position is vastly different in the case of bone, for which Wilson's definitions for “complete” and “average” bone will be adopted here. The former relates to the calcified bone tissue itself, the latter to its soft tissue components, the calculations being based on Spiers' (1951) values for the average dimensions of these soft-tissue spaces.