Necrotizing fasciitis in the neonatal period is a rare, life-threatening condition. Previous cases of neonatal necrotizing fasciitis in the perianal region were speculated to have been initiated by rectal mucosal trauma secondary to rectal temperature measurements. We observed a case of fatal perianal necrotizing fasciitis in a neonate where the process began as a red ring surrounding the anus and guiac-positive stools, detected after a rectal temperature measurement. We speculate that the perianal necrotizing fasciitis that subsequently developed might have been initiated by a minor rectal mucosal injury, and we investigated the instrument used for the rectal temperature measurement to assess any potential areas on the probe sheath cover that might cause a minor injury. Each probe sheath cover examined had three areas that, in our opinion, could possibly generate a minor mucosal injury.