OBJECTIVE: The purpose of this study was to prospectively assess, in the evaluation of patients with suspected malignant pleural mesothelioma (MPM), apparent diffusion coefficient (ADC) values derived from diffusion-weighted images obtained with a free-breathing single-shot spin-echo echo-planar imaging sequence and to correlate the ADC values with the three histologic subtypes of MPM. SUBJECTS AND METHODS: Sixty-two patients with a known pleural abnormality and clinical findings suggestive of MPM underwent diffusion-weighted 3-T MRI and ADC calculation. The pathologic diagnosis was confirmed by surgical procedure. ADC values were correlated with the histologic subtypes of MPM. Statistical analysis was performed with analysis of variance and the Student's t test. RESULTS: Fifty-seven patients had MPM. Forty of the tumors were epithelioid, 11 were biphasic, and six were sarcomatoid. The other five patients had pleural thickening (two patients), metastatic adenocarcinoma (one patient), chronic inflammation (one patient), and malignant lymphoma (one patient). Because of image distortion, the diffusion-weighted images and ADC maps were not satisfactory for assessment in seven cases. The ADC values of MPM were 1.31 +/- 0.15 x 10(-3) mm(2)/s for the epithelioid, 1.01 +/- 0.11 x 10(-3) mm(2)/s for the biphasic, and 0.99 +/- 0.07 x 10(-3) mm(2)/s for the sarcomatoid subtypes of MPM. The ADC of the epithelioid subtype was statistically significantly higher than that of the sarcomatoid subtype (p < 0.05). The ADC in the two cases of benign plaque was 0.85 +/- 0.17 x 10(-3) mm(2)/s. CONCLUSION: The ADC values of epithelioid mesothelioma are higher than those of sarcomatoid mesothelioma. There is no significant difference between the ACD values of biphasic and those of sarcomatoid MPM.