The detailed histological findings of 728 melanoma patients treated at Frenchay Hospital during the period 1967 to 1978 have been fully reviewed and analysed. Of these, 75 patients with lesions less than 0.76 mm thick and 15 with lesions greater than 0.75 mm and less than 0.85 mm thick have been identified using a computerised data base. The general practitioners of all the patients in the series have been approached and the clinical histories found in nearly every case (69/75 and 15/15) thus providing a follow-up period of 5 to 17 years. A number of fatal melanomata have been found in these two groups. The histopathological features have been related to the known prognosis in an attempt to characterise the lethal lesions. No statistically significant relationship could be found between prognosis and any histological feature, including specifically the amount of regression within the lesion. However, it was found that assessment of the ratio of the uninvolved dermis beneath the tumour to tumour thickness itself might prove helpful in future case evaluation.