The handwritten diary of Michał Kazimierz Radziwiłł (1702–1762), also known as Rybeńko, a source familiar to researchers dealing with the history of the 18th century in various fields, is certainly extremely interesting. The diary is kept in the Central Archives of Historical Records in Warsaw (Archiwum Główny Akt Dawnych w Warszawie, AGAD) in the Radziwiłł Archive collection (section VI, sign. II-80). Rybeńko was one of the richest magnates of the Polish-Lithuanian Commonwealth at the time, and this is evidenced by the numerous and extensive estates he owned in the Grand Duchy of Lithuania. He owned about 20 towns, and more than 300 villages, which he inherited from his father and from his father-in-law Janusz Antoni Wiśniowiecki. In view of the above, and concerning the fact that Michal Kazimierz Radziwiłł held one of the highest senatorial offices (from 1744 he was governor of Vilnius, so he was formally in charge of the city that he knew well), did he include information about the capital of the Grand Duchy of Lithuania in the pages of his diary? Being a politically active man who was in possession of such extensive estates, he was constantly on the road (pursuing at the same time a variety of political, economic and social goals), which both his wives complained about. He stayed in many cities of the Polish-Lithuanian Commonwealth, including the capital of Lithuania. Therefore, this article has been written with the intention of presenting the diary as a source for researching one of the most important centres of the Polish-Lithuanian Commonwealth at that time. An attempt is made to consider the circumstances in which entries appear in the diary, whether the prince visited the city regularly or only occasionally, and whether the entries in the diary are systematic reports, or if they are ‘second hand’ information. Finally, the question should be raised as to what type of data was recorded by Rybeńko in the context of Vilnius, and to what extent it can be useful in the study of the city’s history. It also seems crucial to consider the question of what image of the Lithuanian capital emerges from the governor’s records.