Six adults were evaluated for the extent to which they ingest soil. This study was originally part of a large childhood soil ingestion study and was used to validate part of the analytical methodology used in that larger study. The adult study followed the soil tracer methodology of the children's study. The principal findings of the adult study revealed that the four most reliable tracer elements based on recovery studies in a mass balance assessment [aluminum (Al), silicon (Si), yttrium (Y), and zirconium (Zr)] yielded the following mean daily adult soil ingestion estimations: Al (77 mg), Si (5 mg), Y (53 mg), and Zr (22 mg). The median daily soil ingestion estimations based on these same tracer elements were: Al (57 mg), Si (1 mg), Y (65 mg), and Zr (-4 mg). These findings represent the first published quantitative estimates of soil ingestion by adults.