politics. Under the influence of his entourage and Żyron, he decided to support his uncle Mieszko III the Old. He declared Mieszko III his guardian and made a testament in which declared his cousin Mieszko the Younger, son of Mieszko III, his heir. Shortly afterwards Mieszko the Younger started acting as if he was already the ruler of Masovia and Kuyavia, and so Leszek apologized to Casimir II and changed his testament to make Casimir II his heir. On 20 January 1185 Leszek made a big donation to the bishopric of Włocławek, including among others Słońsk and village Kowale. Death and legacy Leszek died in 1186. The day of his death is unknown. The Duke Leszek whose death is mentioned in the necrology of the abbey of St. Vincent in Wrocław on 21 November is, according to historians, Senior Duke Leszek Biały, who was killed on 24 November 1227. There is no information about a wife or children of Leszek of Masovia. Information provided by Kadłubek supported the theory that he died unmarried and childless According to Vasily Tatishchev, 18th-century Russian historian, citing an unknown Polotsk's Latopis, Vasilko Yaropolkovich, Prince of Drohiczyn, was married to daughter of Leszek, duke of Masovia. Older historiography considered this information as reliable, correcting early a crucial fact, namely that Vasilko married a sister, not the daughter, of Leszek. Currently information from Tatishchev about Vasilko Yaropolkovich is considered unreliable. These were not based on lost sources but were likely results of Tatsihchev's fabrications. According to chronicler Jan Długosz, Leszek was buried in the Płock Cathedral. Długosz could have obtained this information from some lost source, but he could also know this from an autopsy, or it could just be his own supposition. However, this information is considered as likely by modern historiography. Under his last will, his heir was Casimir II the Just. Some Polish historians (first Henryk Rutkowski) considered that after Leszek's death, Mieszko III the Old took control over the western part of Kuyavia. However, there is no direct evidence in primary sources about that fact. Footnotes References 1160s births 1186 deaths Dukes of Masovia