Methods and apparatus to mitigate a Denial-of-Service (DoS) attack in a voice over Internet protocol (VoIP) network are disclosed. An example method comprises receiving a communication session initiation message from a communication session endpoint, determining whether the communication session endpoint is associated with a probable DoS attack, and sending to the communication session endpoint a communication session initiation response message comprising a DoS header when the communication session endpoint is associated with the probable DoS attack.