The clinical value of radionuclide images depends on many factors, some controllable and some not. Noncontrollable factors include signal-to-background ratio, contrast gradient, and just-perceptible density difference. Several controllable factors affect the level and proportion of false negative and false positive results: the number of counts in the image, the signal-to-noise ratio, film gamma, contrast, viewing distance, computer and other image manipulation techniques, and selection of the criterion for calling a study abnormal. It is not always true that the more counts an image has, the better it is. For typical clinical situations the number of counts required in an image ranges from 50,000 to 2,000,000, depending on signal-to-noise ratio and film gamma. The clinical value of the images does not necessarily increase as the number of counts increases, the improved resolution and SNR being balanced off by patient motion, field nonuniformity, film nonuniformity, and loss of apparent contrast at high count density. Signal-to-noise ratio varies between two and five in typical clinical situations, and can be improved by better counting statistics. The signal-to-noise ratio is to be distinguished from the signal-to-background ratio, which is not a controllable parameter. Correct viewing distance is an often overlooked parameter. The eye acts as a bandpass filter, perceiving objects with greatest sensitivity when the object occupies between 5 and 10 minutes of visual arc. The correct viewing distance depends on the size of the image being examined, which in the case of Polaroid images varies between 0.1 and 1 cm, corresponding to optimum viewing distances of 30 cm to 3 m. Some organs, such as the liver, may contain filling defects that are smaller than the lower limit of resolution of the imaging system. A technique has been developed in which the observed fluctuation of count rate over the liver is compared with the expected fluctuation (as derived from the number of counts ner unit area). When the observed fluctuation is significantly greater than the expected fluctuation, liver uptake is said to be "nonuniform" despite its appearance to the naked eye. Work is still in progress in this area but initial results have shown a reduction in false negatives in widespread fine metastases and degenerative liver disease in the absence of focal defects as seen on liver scans. Finally, the incidence of false negatives and false positives is affected by the choice of selection criterion by which one decides when to call an image "abnormal." If one knows the approximate distribution of values (such as relative counts per unit area) in normal and abnormal studies, one can maximize the utility of the diagnostic study by selecting an appropriate criterion that is based on the utility of false negative and false positive outcomes. In other words, if the observer is able to formulate a trade-off between false negatives and false positives which would, in his mind, equalize the penalty for being wrong in either case, he can develop a diagnostic strategy that will be reproducible and that will maximize the utility of a test as he sees it. Another observer having a different trade-off can maximize his own utility using a different criterion, and the two observers can estimate what fraction of the normal and abnormal population they would disagree on, based on their separate criteria.