Statistical uncertainties in emission computed tomography were simulated in 60 computer studies involving various numbers of events and distributions of activity. For a uniform disc of activity the rms [root-mean-square] percentage of uncertainty per resolution cell was: 120 .times. (number of resolution cells)1/4 .times. (number of events per resolution cell)-1/2. The more general situation where 1 or 2 regions of uniform activity were surrounded by a uniform background was studied, and for an equal number of recorded events the uncertainties apparently were reduced when activity was concentrated in a portion of the field. The empirical relation rms % uncertainty in nt = 120(N)1/4(nt)-3/4, where nt was the number of events in an average target (organ) resolution cell and N was the total number of events recorded, satisfactorily described relationships between uncertainties, contrast, total number of detected events and number of resolution cells for all 60 computer studies. The theoretical possibility of gated cardiac imaging with 20% uncertainty in 1 cm .times. 1 cm regions and of 1 s cerebral blood-flow images with 20% uncertainty in 2 cm .times. 2 cm regions was shown.