The number of moles (melanocytic naevi) on the body is the strongest known predictor of melanoma risk, yet relatively little is known about the causes and natural history of these common skin lesions. The aim of this study is to assess the contributions of genetic and environmental factors to the development of melanocytic naevi in adolescebnt twins, ascertained through Brisbane primary schools. As part of this study, a custom computer-based image analysis system has been trialed to semi-automate the task of counting and characterising moles.