For cases in which a firearm is not recovered or cannot be associated with a subject, or a fired bullet is too mutilated for physical comparison, two or more bullets may be compared by using the concentrations of selected elements in the lead component. Prior to the current study, researchers had not determined what significance can be placed on a finding of two lead projectiles having indistinguishable compositions. The information needed to assess this significance is the repeat rates of compositional patterns among bullets. The study used a data set from 1,837 samples from a sampling of 23,054 lead bullets and shot pellets received as evidence in the FBI Laboratory from 1989 through 2002. The method used to select samples from case submissions ensured that no two samples of the same general type from the same case were included and that no bias was introduced concerning the representation of manufacturers or production sources. A total of 1,686,366 pairwise lead sample comparisons were made by using the concentrations of the elements of arsenic, antimony, bismuth, cadmium, copper, silver, and tin, using a match criterion of 2 times the sum of the standard deviations of the paired samples. Of the 1,837 samples, 1,397 samples (76 percent) were distinguishable from every other sample in the study. The total number of indistinguishable sample pairs was 674, a frequency of 1 out of every 2,502 comparisons. The frequency of occurrence of matching samples decreased as the number of measured elements was increased and as the precision of the measurements improved. For bullets in which all 7 elements were determined, the match frequency was 1 in 7,284. Thus, compositional comparison of bullet lead provides a reliable, significant point of evidentiary comparison of potential sources of crime-related bullets. 7 tables, 4 figures, and 20 references