Abstract: The subject of the study is the decrypted telegrams of Swedish diplomatic missions sent to the Minister of Foreign Affairs of the Swedish Kingdom K. Wallenberg, intercepted by the special services of the General Staff of the Russian Army at the end of 1915-1916. Sweden, while remaining a formally neutral power, nevertheless not only closely followed the events on the fronts of the First World War, but also conducted active trade and provided transit services to both Germany and Russia, deriving huge economic benefits from this situation. The documents of this period mainly cover the events in the Balkans, where the allied forces of the Entente tried to deploy the so-called Salonika (Macedonian) front to help the Serbian army and to divert enemy forces from other fronts. Based on the methods of historicism and systematic research, the reports of diplomats who informed their leadership about the events around Thessaloniki, about the situation and actions of the allied forces and the troops of the Central Powers, about the further intentions of the opposing forces, about the fate of Serbia and Montenegro, express assessments and opinions on specific issues and the prospects for the development of the war. Since there were supporters of both the Entente and the Austro-German bloc in the Swedish elite, the following conclusions can be drawn from the documents under consideration. They allow us to assess the range of issues of the most interest to Swedish diplomats and the Swedish government, as well as the degree of their awareness, the quality of information sources and the level of understanding of the content of the events taking place. These documents are very important for clarifying Sweden's position during the First World War. Since there were supporters of both the Entente and the Austro-German bloc in the Swedish elite, it can be understood from the documents under consideration that during this period it became increasingly obvious to Swedish diplomats that despite all the difficulties, including in the Balkans, the preponderance of the Entente countries was more likely. Most of the analyzed archival materials are being introduced into scientific circulation for the first time.