The most reliable and practicable measure in assuring coexistence in respect to pollen-mediated gene flow from genetically modified (GM) to conventional maize (Zea mays L.) is an isolation distance separating GM and non-GM fields. Therefore, we tested distances between 24 and 102 m at three sites in northern Germany using a field orientation representing a worst case scenario concerning wind direction. During the 3 yr of field trials the highest levels of gene flow occurred at the site and year with the longest flowering synchrony and the strongest wind blowing constantly from the GM to the non-GM field. It was shown that the GM content of a neighboring non-GM maize field is mainly determined by wind speed and direction as well as by non-GM maize field depth. Based on the maximum outcrossing data obtained it can be concluded that for non-GM maize fields being 200 m deep or more an isolation distance of 50 m is sufficient to keep the GM content of the total fields grain harvest below the European Union (EU) labeling threshold of 0.9%. However, non-GM grain maize fields with smaller field depth require larger isolation distances or additional coexistence measures. In most cases discarding 6 m of the GM maize facing non-GM maize field edge has proven to be such a valuable measure. In silage maize production 50 m isolation distance is adequate even for non-GM maize field depths down to 50 m. We recommend flexible separation distances in dependence on non-GM maize field depth to comply with EU coexistence requirements.