Compare apparent diffusion coefficient (ADC) values between benign and malignant mass lesions in a cohort of children referred for imaging of a mediastinal mass.Prospective study including 24 consecutive children (11 boys, 13 girls aged 5 months to 16 years). All underwent echo planar diffusion weighted MR imaging of the mediastinum with b-factors of 0 and 600 s/mm(2). Apparent diffusion coefficient (ADC) values were calculated and correlated with the surgical finding or biopsy.The mean ADC value of malignant mediastinal tumors was 0.91 (S.D., 0.17) × 10(-3) mm(2)/s and of benign lesions 1.8 (S.D., 0.33) × 10(-3) mm(2)/s. There was significant different in the ADC value between malignant tumors and benign mediastinal tumors (P<0.001). Selection of 1.2 × 10(-3) mm(2)/s as a threshold value for differentiating malignant from benign mediastinal masses has an accuracy of 93%, sensitivity of 92%, specificity of 94%, positive predictive value of 94%, negative predictive value of 92% and area under the curve of 0.962.Apparent diffusion coefficient value is a promising non-invasive parameter for assessment of mediastinal mass in children.