Unidimensional Response Evaluation Criteria in Solid Tumor (RECIST) has been recently proposed in the attempt to simplify the standardized bidimensional World Health Organization (WHO) criteria. The complete accord between these two measurement systems was established in a large comparative study [J. Natl. Cancer Inst. 92(3) (2000) 205] that demonstrated the validity and the good performance of RECIST criteria. We report four cases of inadequacy of RECIST criteria in the evaluation of response to chemotherapy in patients with malignant pleural mesothelioma. These four patients were enrolled in two consecutive multicenter phase II clinical trials investigating the activity of a novel chemotherapy regimen in advanced pleural mesothelioma. They were judged as having an objective response to chemotherapy according to WHO criteria. Reassessed according to both methods, we found that results obtained with RECIST criteria do not correspond to WHO underestimating response to chemotherapy. Our data raise doubts about the applicability of unidimensional RECIST response criteria to mesothelioma and, possibly, to any tumor involving the chest wall.