Tumour thickness is the most important prognostic factor in malignant melanoma. To reduce the melanoma-related mortality, factors related to the presentation of thick melanoma have to be identified. Three samples of melanoma patients (n=694) were studied for this purpose. Histogenetic type was the only factor which differentiated between 'thin' (= 0.8 mm) and 'thick' (> 2.0 mm) lesions. During a 10-year period only 3% of the nodular lesions were 'thin' at diagnosis. Differences in knowledge about melanoma or the location of the lesion (either 'easy' or 'difficult' for the patient to observe) did not explain differences in tumour thickness. The most common tumour site irrespective of histogenetic type and gender was 'back of the trunk'. 'Increase in diameter' and 'bleeding' were the symptoms most frequently reported by patients with 'thick' melanoma. 'Thick' lesions were diagnosed in older age groups and in men to a greater extent. Considering these results, melanoma prevention should also be targeted to older age groups and attention should be paid to symptoms such as 'increase in diameter' even in the absence of other characteristic symptoms of melanoma. An increased proportion of nodular melanoma diagnosed as 'thin' lesions can be interpreted as a step forward in secondary prevention.