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Background. Klippel-Feil syndrome (KFS) is characterised by congenital vertebral fusion of the cervical spine and a wide spectrum of associated anomalies. KFS has often been considered a sporadic syndrome. However, since the publication of the original KFS classification early this century, a number of KFS families have indicated heterogeneity complicated by a broad range of variable expression. Objective. The two major objectives of this study were (1) to identify differences and similarities in the postnatal appearance, morphology, position and inheritance of vertebral fusions within and between KFS families and (2) to establish a new KFS classification focussed on KFS aetiology. Materials and methods. Vertebral fusions were assessed via spinal radiography. Chromosomal karyotypes were performed using routine cytogenetics. Results. The medical histories of three KFS families are presented. The postnatal time, position and appearance of vertebral fusions, associated anomalies and mode of inheritance were different for the three KFS families. Four classes of KFS are described in a comprehensive classification table that allays much of the uncertainty arising from KFS heterogeneity and variable expression. Conclusion. We have described four different KFS classes (KF1–4) within a comprehensive classification that addresses KFS genetic heterogeneity. The position of vertebral fusions in the cervical spine and their incidence within affected families are delineating features of KFS.