Melanomas are more likely to develop on the left side of the body, but scientists are at a loss to explain why.
An analysis of cancer data from Australia and other developed nations has revealed that 10 per cent more invasive tumours are reported on the skin of left legs, arms and side of the body than on the right.
The researchers say the curious trend is a mystery, but it is probably not a coincidence.
Scottish cancer researcher David Brewster and colleagues worldwide studied the records in cancer registries in Australia, Europe, Britain and the US over five years.
They found no differences in sex or age groups affected, or for upper limb versus lower limb melanomas.
But they found this odd imbalance for sides of the body affected.
"The excess of left-sided tumours seems unlikely to be explained by chance or recording bias," they wrote in the European Journal of Cancer.
They ruled out the likelihood it was caused by one arm being exposed while driving, sun-baking behaviour or uneven sunblock application.
The team also ruled out that it could be sparked by an uneven distribution of melanocytes, a pigment-producing cell that helps to protect the body from ultraviolet light.
The results add to growing evidence that many cancers, particularly lung cancer, seem to favour one side of the body over the other.
Australia's melanoma rates have jumped 18 per cent for men and 21 per cent for women in the past 10 years. It is the most common cancer in men aged 25 to 54 and in women from 15 to 29.
Other new research shows that Australians are about four times more likely to develop melanoma than people in Britain, but they are also more likely to survive the disease.
Researchers at the Cancer Council NSW analysed cases in both countries and said that melanoma rates are on the rise in both countries.
Most melanomas diagnosed in Australia were in the early stage but British patients were picked up later and received a poorer prognosis.