International co-authorship is generally thought and often found to have positive effects on the citation rate of scientific publications. We study the effect quantitatively in the example of four major and two medium Hungarian universities. It was found that the positive effect of international cooperation on citation impact was not limited either to a small set of highly cited papers or a narrow range of highly influential countries, although both the highest cited papers and the "scientific superpowers" had emphatic significance in determining the incremental citation impact values. Although Hungarian institutions are recently mainly encouraged to cooperate with EU partners, the USA and even countries from the Far-East and the Pacific region proved to be successful cooperating partners, as well. The conclusions may be generalized to other countries of similar international status.