Malignant mesothelioma is difficult to distinguish from other pleural malignancies and also from benign mesothelial lesions. A morphometric study has been performed to distinguish between them using quantitative size and shape parameters. Seven cases of malignant mesothelioma, 5 cases of pleural metastatic adenocarcinoma and 4 cases of benign mesothelial lesions were selected and subjected to S.A.M. (Shape Analytical Morphometry). The results, statistically evaluated, showed that morphometric parameters can be proposed for diagnostic purposes being useful in the discrimination among the three populations. In fact, multivariate discriminant analysis (MDA) of the quantitative parameters obtained by morphometrical study distinguished the three groups of lesions with only 2% of error between BML/MM, 7% of error between BML/MA and 25% between MM/MA.