Yoshitatsu was quite possibly the biological son of Toki Yorinari (Yoshiyori), whom Saitô Dôsan (Toshimasa) went on to usurp in 1544. According to one story, Dôsan had in fact won Yoshiyori's concubine through a wager: Dôsan bet Yorinari that he could put a spear through the eye of a tiger painted on a sliding door. Dôsan succeeded and was given the lady as a concubine. Yoshitatsu was born seven months later and his parentage would come to be questioned. At first, it is said Dôsan doted on him and he was duly established at Inokuchi (Inabayama) Castle while his father retired and took up residence nearby in Sagiyama. Rumors that Yoshitatsu was in fact not Dosan's real son (that is, that he was actually Yorinari's son) persisted, however, and Dôsan apparently began to think of naming one of his other sons as heir. Yoshitatsu had come to suspect his father's intentions. Though he actually did suffer from leprosy, he feigned illness and arranged for the murder of two of Dôsan's natural sons in 1555. The following year he and his father met in battle at the Nagara River. Yoshitatsu emerged triumphant after a stiff fight, and took control of Mino. He proved a capable commander and was able to defeat attempts by Oda Nobunaga to avenge Dôsan's death, but died of his illness in July 1561.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005