L1 syndrome is a group of overlapping, X-linked disorders caused by mutations in L1CAM. Clinical phenotypes within L1 syndrome include X-linked hydrocephalus with stenosis of the aqueduct of sylvius (HSAS); mental retardation, adducted thumbs, shuffling gait, and aphasia (MASA) syndrome; spastic paraplegia type 1; and agenesis of the corpus callosum. Over 200 mutations in L1CAM have been reported; however, only a few large gene deletions have been observed. We report on a 4-month-old male with a de novo whole gene deletion of L1CAM presenting with congenital hydrocephalus, aqueductal stenosis, and adducted thumbs. Initial failure of L1CAM gene sequencing suggested the possibility of a whole gene deletion of L1CAM. Further investigation through chromosome microarray analysis showed a 62Kb deletion encompassing the first exon of the PDZD4 gene and the entire L1CAM gene. Investigations into genotype-phenotype correlations have suggested that mutations leading to truncated or absent L1 protein cause more severe forms of L1 syndrome. Based on the presentation of the proband and other reported patients with whole gene deletions, we provide further evidence that L1CAM whole gene deletions result in L1 syndrome with a severe phenotype, deletions of PDZD4 do not cause additional manifestations, and that X-linked nephrogenic diabetes insipidus reported in a subset of patients with large L1CAM deletions results from the loss of AVPR2.