The value of rhodium filtration in mammography was compared with that of conventional molybdenum; dosimetric and qualitative tests were performed on a phantom and in vivo on informed and consenting patients. Phantom dosimetric tests confirmed a dose reduction with rhodium filtration changing according to thickness and primary beam energy. With rhodium filtration the dose decreased by 40% on the average topping 67% in breasts thicker than 6.5 cm. No significant differences were observed between rhodium and molybdenum filtration in clinical and phantom qualitative tests. Four radiologists--three of them in double blind--studied 29 mammograms repeated in the same technical setting and expressed a very slight preference for molybdenum also in thick and dense breasts. The slight preference was not based on higher diagnostic yield and therefore clinically irrelevant. Preferences appeared to be subjective, differing from one radiologist to another. In conclusion, the authors suggest the use of rhodium filtration in thick breasts because of the lower administered dose and of shorter exposure time with direct magnification.