The prognostic significance of 2 histological parameters, level of invasion and maximum thickness is evaluated in 248 cases of malignant melanoma of the limbs staged T1-3NoMo which were collected for Trial No. 1 of the W.H.O. Collaborating Centres for the Evaluation of Methods of Diagnosis and Treatment of Melanoma between September 1967 and December 1974. There is a linear relation of tumor thickness to mortality with a high statistical significance (P = 0.0002). Mortality also increases with progression of the level of invasion. The incidence of occult metastases to the regional lymph nodes increases with increasing thickness or level of invasion. Moreover the age and sex corrected survival curves are also dependent on both parameters. The comparison of the 2 methods revealed that maximal tumor thickness is a more powerful measure of prognosis than is the determination of the level of invasion.