This movie started when Studio Ghibli received a proposal from a Japanese theme park to make a twenty-minute animation starring cats.
This was originally going to be a short straight-to-video project, but producer Toshio Suzuki was so impressed with director Hiroyuki Morita's storyboards, and especially the main character of "Haru", that he encouraged Planner Hayao Miyazaki to give this a cinematic release.
So far, this is Studio Ghibli's only spin-off or sequel feature film.
Took three years to complete.
One recurring joke in the original Japanese dub is that Muta keeps being mistakenly called "Buta", which is Japanese for "pig". Translators were unable to work this joke into the English dub; the nearest they came was for Haru to call him "Moo-ta", and have him think she was calling him a cow.