### Objective: Pigmented orthochromatic leukodystrophy (POLD) and hereditary diffuse leukoencephalopathy with axonal spheroids (HDLS) are rare neurodegenerative disorders characterized by cerebral white matter abnormalities, myelin loss, and axonal swellings. The striking overlap of clinical and pathologic features of these disorders suggested a common pathogenesis; however, no genetic or mechanistic link between POLD and HDLS has been established. Recently, we reported that mutations in the colony-stimulating factor 1 receptor (*CSF1R*) gene cause HDLS. In this study, we determined whether *CSF1R* mutations are also a cause of POLD. ### Methods: We performed sequencing of *CSF1R* in 2 pathologically confirmed POLD families. For the largest family (FTD368), a detailed case report was provided and brain samples from 2 affected family members previously diagnosed with POLD were re-evaluated to determine whether they had HDLS features. In vitro functional characterization of wild-type and mutant *CSF1R* was also performed. ### Results: We identified *CSF1R* mutations in both POLD families: in family 5901, we found c.2297T\>C (p.M766T), previously reported by us in HDLS family CA1, and in family FTD368, we identified c.2345G\>A (p.R782H), recently reported in a biopsy-proven HDLS case. Immunohistochemical examination in family FTD368 showed the typical neuronal and glial findings of HDLS. Functional analyses of *CSF1R* mutant p.R782H (identified in this study) and p.M875T (previously observed in HDLS), showed a similar loss of *CSF1R* autophosphorylation of selected tyrosine residues in the kinase domain for both mutations when compared with wild-type *CSF1R*. ### Conclusions: We provide the first genetic and mechanistic evidence that POLD and HDLS are a single clinicopathologic entity.