The school authorities did approve the use of their school name in the movie without hearing the story. Later they demanded it removed after watching the movie. Because it is about two lovers eloping and claims that it is a true story.
The writer director claims and is written on the end credits that the movie is (with names and many changes) inspired by a real life incident he heard during one of his train travels. But he did not produce any evidence for that.
Kaadhal was remade in Kannada as Cheluvina Chittara in 2007, in Bengali as Chirodini Tumi Je Amar in 2008, in Bengali Bangladesh as Nogor Mastan in 2015, in Marathi as Ved Laavi Jeeva in 2010, in Nepali as Manjari in 2013 and in Punjabi as Ramta Jogi in 2015.
The film was initially turned down by actors Dhanush and Shanthanu Bhagyaraj, Telugu actor Ram also auditioned for the lead role but was unsuccessful. After considering both Manikandan and Bharath from the cast of Boys (2003), Shankar chose the latter to star in his production.
For the female lead, the makers held discussions with Ileana D'Cruz and then Varalaxmi Sarathkumar, but the latter's father was reluctant to let her become an actress at the time. The team then picked Saranya Nag who was in Class IX, when cinematographer Vijay Milton referred her on to Balaji Sakthivel. She was initially considered to play the heroine in the film, but the role was later handed to Sandhya, after the director felt she looked too young.