Elementary theory underlying the relationship between the number of scatterers per resolution cell(N)and echo intensity signal-to-noise ratio (SNR) is reviewed. A relationship between the probability density functions for estimates ofNandSNR2is derived. This relationship is validated using a computer simulation. Phantom andin vitroexperiments are described. In one set of experiments on phantoms, empirical distributions of estimates ofNandSNR2are measured and compared to theoretical predictions. The utility ofSNR2for discrimination of phantoms with different values forNis assessed using receiver operating characteristic (ROC) analysis. In another set of experiments, the frequency dependence of theSNR2estimate is investigated for a two-component phantom and for excised dog kidney. It is shown that the frequency dependence of the SNR can help to identify the presence of two or more scattering components that are spatially mixed. With regard to kidney data, measurements performed both parallel and perpendicular to the predominant nephron orientation are reported. The observed anisotropy is compared to the anisotropy of backscatter coefficient encountered in previous investigations.