The unique growth pattern of malignant pleural mesothelioma (MPM) presents challenges for clinical investigators assessing tumour response to treatment. Assessment of response is an important surrogate for patient benefit, particularly in phase II clinical trials. Neither the previous WHO criteria nor the more recent RECIST criteria are well suited to tumour measurement in this disease. This paper discusses uni-dimensional measurement of tumour response in pleural mesothelioma, and the development and validation of the modified RECIST criteria for MPM, which attempt to overcome some of the shortcomings of the RECIST criteria in this setting.