Two methods of evaluating bone scintigrams in Paget's disease of bone were compared. Bone scintigraphy was performed after IV injection of 20 mCi 99mTc-Sn-EHDP. Qualitative evaluation was carried out by visual assessment of the photographs of the oscilloscope of the gamma camera, using a scale of six well-defined scores. Quantitative evaluation of uptake of radionuclide was obtained by calculating the ratio of the number of counts per pixel in pagetoid bone with that in comparable normal bone; measurements were performed by a computer interfaced to the gamma camera, after marking the regions of interest with a light-pen. In 42 patients with untreated Paget's disease there was a linear relationship between the visually assessed score and the logarithm of the computer-measured ratio. In 27 patients treated with APD a decrease in uptake was measured in all lesions and was usually also detected visually; however, in 14% of these lesions the changes (ranging from 9% to 70%) were not visible to the eye. Here too, visual estimation of the changes corresponded to the logarithm of the measured changes. In conclusion, there is a semilogarithmic relationship between the visual and the physical methods.