This study was undertaken to determine the extent to which area measures of posterior fossa structures can be used confidently to represent structure volumes. MRI scans were obtained from three groups: fragile X males, males with other developmental disabilities, and males with normal IQ. The areas of the midbrain, pons, medulla, cerebellar vermis, and fourth ventricle were measured in midsagittal sections. Volumes of midbrain, pons, medulla, cerebellum, fourth ventricle, and third ventricle were obtained by measuring these structures in contiguous axial slices. In addition, the largest axial area for each structure was identified. Analysis revealed that midsagittal area measures for pons, medulla, and fourth ventricle were significantly correlated with structure volumes, and that all of the largest axial area measures were significantly correlated with structure volumes. However, only the midsagittal area measure of fourth ventricle and the largest axial area measures of fourth ventricle and cerebellum were correlated with volume measures with an r of .80 of greater. Results of this study suggest that area measures may not accurately represent three-dimensional structure size.