Germany is the main trading partner for Poland. There are direct and indirect effects of international connections. This article verifies the hypothesis about the dominant influence of direct connections on the Polish and German economies. The article examines the usefulness of tests for linear and nonlinear causalities and cospectral analysis in determining the direction of economic flows. The research results confirm the high level of synchronization of business activities between Poland and Germany. Causality constitutes only few percent of the variance of the annual growth rate for industrial production and is much stronger in the Polish economy.