Research suggests synaesthesia is nearly three times as common in adults with autism spectrum disorder than in the general population.
The two conditions may share common features such as unusual wiring of the brain, say UK scientists.
The study helps understanding of how people with autism experience life, says the National Autistic Society.
Synaesthesia is a condition where one sense automatically triggers another. Some people experience tastes when they read or hear words, some perceive numbers as shapes, others see colours when they hear music.
People with synaesthesia might say: "The letter q is dark brown," or: "The word 'hello' tastes like coffee," for example. People with the condition [autism] can find everyday life confusing or even frightening so research like this, which helps us to understand more about how they experience the world, is valuable”
Carol Povey, National Autistic Society
Following anecdotal evidence of links between synaesthesia and Asperger's syndrome, researchers at the Autism Research Centre at Cambridge University set out to test the idea.
More than 200 study participants - 164 adults diagnosed with high-functioning autism or Asperger's syndrome, and 97 adults without autism - were asked to fill in questionnaires to measure synaesthesia and autism traits.
The study found one in five adults with autism spectrum conditions - a range of related developmental disorders, including autism and Asperger's syndrome - had synaesthesia compared with about 7% of people with no signs of the disorders.
Simon Baron-Cohen, who led the research, told reporters: "Synaesthesia involves a mixing of the senses and it's a very subjective private experience, so the only way we know it's happening is if you ask people to report on their experiences.
"And what this new study has done is ask people whether they experience synaesthesia, for example where a sound triggers the experience of colour or a taste triggers the experience of colour, and finding that these unusual experiences are actually much more common in autism than we previously knew."
The research, to be published in the journal Molecular Autism, suggests that while the two conditions might appear distinct, there could actually be some underlying similarities in brain connectivity.
More information: Is synaesthesia more common in autism? Authors: Simon Baron-Cohen, Donielle Johnson, Julian Asher, Sally Wheelwright, Simon E Fisher, Peter K Gregersen, Carrie Allison. In Molecular Autism (November 1st, 2013). www.molecularautism.com/content/4/1/40