It's based on the web series created by Norberto Ramos del Val. "Hienas" was a no-budget project for the Internet, but after a year and a half since the premiere of the first episode, the distribution company Friki Films, offered the director the possibility to edit all the episodes as a feature-length movie. Norberto Ramos del Val accepted the proposition.
None of the people involved in the making of "Hienas" earned a salary. Everyone of them participated as a favor for Norberto Ramos del Val, with whom they worked previously.
Manuela Velasco was originally slated to appears as Ana, the psychology that helps 'Ángela Boj' character. She was cast before the success of REC (2007). After she won the Goya Award, she couldn't continue working in Hienas (2009) because of her busy work schedule (she was attached to star in La chica de ayer (2009)), and she had to be replaced by Irene Rubio, who appears playing the same character in a brief scene near the end of the movie.