Abstract Acquirers and targets in horizontal mergers are typically multiproduct firms that often sell the same core product. How do mergers affect these multiproduct firms? Using Danish data, we establish empirically that the combined domestic product scope of the acquirer and target falls after a merger, and sales of the acquirer become more concentrated towards the core product. Moreover, the acquirer exports more products to more destinations that are costly to serve. To rationalize these findings, we develop a model of mergers between oligopolistic firms, highlighting merger-induced within-firm adjustments along the product dimension that promote efficiency improvements and international trade.