AbstractThis paper extends previous theorems concerning aggregation bias in single‐region and national input‐output models to interregional input‐output (IRIO) models. The results indicate that the conditions for zero aggregation bias are more stringent in input‐output models involving multiple regions. In addition, the paper derives the relationship between the information content of IRIO and national input‐output accounts. The results confirm the conventional wisdom that the information content of multiple region input‐output models of the U.S. economy contain more information than national input‐