OBJECTIVE: On diffusion-weighted magnetic resonance (MR) images, posterior fossa tumors may exhibit the various signal intensity and apparent diffusion coefficient (ADC) reflecting their histology and cellularity. The purpose of this study was to evaluate the relationship between ADC and tumor cellularity in medulloblastoma and other posterior fossa tumors. METHODS: Pre-operative diffusion-weighted MR images were retrospectively reviewed in 26 patients with posterior fossa neoplasms: 11 medulloblastomas, one atypical teratoid/rhabdoid tumor (AT/RT), four glioblastomas, four ependymomas, three pilocytic astrocytomas and three hemangioblastomas. The minimum ADC (minADC) value of each tumor was determined on ADC maps derived from isotropic diffusion-weighted MR images. The minADC values were compared by a two-tailed t-test. Tumor cellularity measured in surgical specimens was compared with the minADC value by simple linear regression analysis. RESULTS: The mean minADC value of the medulloblastoma was significantly lower than those of ependymoma, pilocytic astrocytoma and hemangioblastoma without overlap in the range of minADC values. Therefore, medulloblastomas could be clearly differentiated by absolute minADC values. AT/RT and glioblastoma had similar minADC values to medulloblastoma. Tumor cellularity was negatively correlated with the minADC value in medulloblastomas and other posterior fossa tumors. DISCUSSION: The low minADC value of medulloblastomas reflects the high tumor cellularity. Analysis of ADC values has high predictive value for the differentiation of medulloblastoma from other posterior fossa tumors.