Having more than 11 moles on one arm indicates a higher-than-average risk of skin cancer or melanoma, research suggests.
Counting moles on the right arm was found to be a good indicator of total moles on the body. More than 100 indicates five times the normal risk.
The study, published in the British Journal of Dermatology, used data from 3,000 twins in the UK.
GPs could use the findings to identify those most at risk, it said.
Melanoma is a type of skin cancer affecting more than 13,000 people in the UK each year.