Hypomyelination syndrome of the Weimaraner dog is a disease characterised by a reduction or absence of myelin in the axons of the central nervous system (CNS) exclusively. The objective of this study was to analyse the cause of this deficiency of myelin. Tissue samples of the CNS of three Weimaraner dogs with neurological signs were fixed in 10% formalin and embedded in paraffin wax, and histochemical, immunohistochemical and ultrastructural studies were performed. Histochemical staining with haematoxylin and eosin and Kluver-Barrera techniques showed generalised pallor in the peripheral areas of the ventral and lateral funiculi of the spinal cord. Immunohistochemical analysis showed a weak expression of both proteolipid protein (PLP) and myelin basic protein (MBP) and a marked decrease of Olig2+ cells in the demyelinated areas. The immunohistochemical findings suggested a myelination or remyelination failure because of the smaller population of oligodendrocytes. However, PLP gene mutations may also be the cause of the decrease of PLP expression as described in other species. © 2010 British Small Animal Veterinary Association.