The lesion detectability with a scintillation camera as a function of count density was determined for the four spherical cold lesions with diameter 0.9-2.5 cm (in square root 2 step) within a uniform background activity. It can be seen from the curves that the lesion detectability increases with an increase in count density until 2 kcounts/cm2, however it does not appreciably improve for count densities above 2 kcounts/cm2. The curves also show that, for a giving count density, the lesion detectability decreases as the lesion size or object contrast decreases. From these curves minimum count density required for recognition of each cold lesion is deduced. This value is applied to the evaluation of scintillation camera system.