The prognosis of melanoma of the skin has been investigated in many studies. In this report papers on the prognosis of stage I melanoma published since 1975 in the leading oncology journals have been reviewed. Further the data collected by the WHO Collaborating Centre for Evaluation of Methods of Diagnosis and Treatment of Melanoma are analysed, and the results compared with those of other series. Three factors emerge as clearly influencing prognosis: sex, maximum tumour thickness and ulceration. The role of other factors and particularly that of an interaction between tumour thickness and ulceration conjectured by some authors remains questionable.