The phenotype of severe congenital hydrocephalus secondary to neural cell adhesion molecule L1 (L1CAM) gene mutations includes the distinct finding of brainstem corticospinal tract hypoplasia. Using diffusion-weighted imaging (DWI), we failed to demonstrate anisotropy in the corticospinal tracts of the basis pontis in 4 affected boys with L1CAM mutations. The DWI findings correlated with the neuropathological findings in a fifth patient. DWI may be a useful technique to screen for boys with L1CAM mutations.