The aim of this study was to assess the potential role of tumor diameter in explaining variations in tumoral invasion and in the initial prognosis for patients with malignant melanoma (MM). This was a multicenter, cross-sectional study that recruited between 2000 and 2009 patients with primary in-situ MM (Tis) and invasive cutaneous MM. Tis and MMs with a Breslow's thickness less than 1 mm (T1) were grouped (Tis-T1) and tumors with a Breslow's thickness 1 mm or more were also analyzed in combination (T2-T4). The tumor size was measured after routine formalin tissue fixation. Primary outcomes were the correlation between Breslow's thickness and tumor size, and the role of tumor size in explaining variations in Breslow's thickness, as assessed by the Pearson correlation test and logistic binary regression with calculation of the odds ratios. A total of 1610 MM patients were included and analyzed. The Pearson correlation between tumor size and Breslow's thickness was 0.42, with a determination coefficient of R2=0.18 (P>0.01). Correlations between tumor size and thickness were stronger in patients aged 30-60 years (r=0.42, R2=0.1764, P<0.001) and in tumors arising on the upper limbs (r=0.55, R2=0.3025). The odds ratio of identifying a T2-T4 stage MM in patients with tumors larger than 1 cm in size was 2.76 (95% confidence interval 2.25-3.39, P<0.001). Even though a direct, positive, and strong association between tumoral size and Breslow's thickness might be expected in melanoma cases, the strength of this association has been moderate. Tumor size explains a low burden of the variation observed in the Breslow's thickness.