SUMMARYWe derive a quantity‐based structural gravity equation system in which both trade flows and error terms are cross‐sectionally correlated. This system can be estimated using techniques borrowed from the spatial econometrics literature. To illustrate our methodology, we apply it to a well‐known Canada–US trade dataset. We find that border effects between the USA and Canada are smaller than suggested by previous studies: about 7.5 for Canadian provinces and about 1.3 for US states. Hence controlling directly for cross‐sectional interdependence among both trade flows and error terms reduces measured border effects by capturing ‘multilateral resistance’. Copyright © 2010 John Wiley & Sons, Ltd.