OBJECTIVE To present case reports of two siblings with primary hypomagnesaemia both presenting with seizures, and one also with a cardiac arrhythmia. To briefly review the pathophysiology, clinical features, diagnosis, management and genetics of this disorder. METHODS Published literature reports of primary hypomagnesaemia and studies of hypomagnesaemia in humans. Reports of the clinical features and inheritance of primary hypomagnesaemia. RESULTS The information is descriptive of the pathophysiology, clinical features, diagnostic criteria, and management. Considered modes of inheritance are presented. Two cases of primary hypomagnesaemia in brothers of consanguineous parents are described. Cardiac arrhythmia at presentation has not previously been reported. Diagnosis and adequate magnesium supplementation controls the biochemical disorder and the neurological development is normal. CONCLUSIONS Primary hypomagnesaemia should be considered in infants with seizures, as failure to identify this metabolic disorder can result in death. Subsequent siblings, particularly male, should be closely monitored.