There is a theory that Mitsunari plotted the poisoning of Ujisato GAMO, Gamo-sodo (the Gamo Family feud) and the resultant weakening of the Gamo Family, but many of Gamo's former retainers served and died for Mitsunari so today this theory is denied.
蒲生氏郷を毒殺し、後に蒲生家の騒動（蒲生騒動）及び蒲生家の弱体化を三成が謀ったとも言われたが、蒲生家の多くの旧臣が三成に仕え、三成のために死んでいるという反証もあり、現在は否定されている。