Inside the EU, the commercial integration of the CEE countries has gained remarkable momentum before the crisis appearance, but it has slightly slowed down afterwards.Consequently, the interest in identifying the factors supporting the commercial integration process is high. Recent findings in the new trade theory suggest that FDI influence the trade intensity but the studies approaching this relationship for the CEE countries present mixed evidence, and investigate the commercial integration of CEE countries with the old EU members. Against this background, the purpose of this paper is to assess the CEE countries' intra-integration, focusing on the Czech Republic, Hungary, Poland and the Slovak Republic.For each country we employ a panel gravitational model for the bilateral trade and FDI, considering its interactions with the other three countries in the sample on the one hand, and with the three EU main commercial partners on the other hand. We investigate different facets of the trade -FDI nexus, resorting to a fixed effects model, a random effects model, as well as to an instrumental variable estimator, over the period 2000-2013. Our results suggest that outward FDI sustains the CEE countries' commercial integration, while inward FDI has no significant effect. In all the cases a complementarity effect between trade and FDI is documented, which is stronger for the CEE countries' historical trade partners. Consequently, these findings show that CEE countries' policymakers are interested in encouraging the outward FDI toward their neighbour countries in order to increase the commercial integration.with a trade-off between focusing on specific countries for which data are in general available for a longer period, or including in the sample more CEE countries, but reducing thus the 3 number of observations. We therefore chose the first option and we test the trade -FDI nexus for the Czech Republic, Hungary, Poland and the Slovak Republic (CEE-4), using Organisation for Economic Co-operation and Development (OECD) statistics for the period 2000-2013. The OECD statistics contain information about the bilateral investments between the OECD members. Six CEE countries are OECD members, namely the Czech Republic, Estonia, Hungary, Poland, the Slovak Republic and Slovenia. However, Slovenia and Estonia are excluded from our sample because of insufficient number of observations. Moreover, we are forced to start with the year 2000 as no previous data are available for the Slovak Republic's inward FDI before this year 1 . The selected countries (to a smaller extent Poland), represent the countries with the highest level of trade openness in the CEE group of countries. According to the World Bank statistics (World Development Indicators), over the period 2000-2013, the trade openness in CEE-4 is above the average level of the entire group of CEE countries (118% compared to 103%), and considerably above the EU level (72%). At the same time, CEE-4 is historically considered as representing the advanced group of CEE countries ...