Preferential trade agreements (PTAs) are characterized by liberalization with respect to only a few partners and thus they can potentially clash with, and retard multilateral trade liberalization (MTL). Yet there is almost no systematic evidence on whether the numerous existing PTAs actually affect MTL. We provide a model showing that PTAs hinder MTL unless they entail accession to a customs union with internal transfers. Using product-level tariffs negotiated by the European Union (EU) in the last two multilateral trade rounds we find that several of its PTAs have clashed with its MTL.