Since the interwar period, the Swedish economy has been dominated by a few business groups that control practically all Swedish industry of any importance. This article provides a sociologically informed account of the existence and lasting dominance of business groups in Sweden. In challenging efficiency‐based accounts of business groups in Sweden offered by transaction‐cost economists, I adopt a historical and dynamic approach that understands economic institutions not as efficient responses to external circumstances, such as market conditions, but as “social constructions.” This approach emphasizes the role of contingency as well as agency in the formation and reproduction of institutions. The article demonstrates the historical contingencies of the emergence of the business groups—in the form of economic crises in the interwar period—and how the groups became institutionalized in the postwar period through a process of social exchange among the key actors in the Swedish political economy: the business elite, the trade unions, and the state.