L-Arginine:glycine amidinotransferase (AGAT) deficiency is a very rare type of creatine deficiency syndrome characterized by global developmental delay, intellectual disability, and myopathy. AGAT deficiency is characterized by global developmental delay, appearing in infancy, which can be associated with language impairment and autistic behavior in some, as well as a mild to moderate intellectual disability. Progressive muscle weakness and fatigability have been reported in older patients. Seizures and failure to thrive have also been described. If creatine supplementation is administered early enough, psychomotor delay may be avoided. AGAT deficiency is caused by mutations in the L-arginine:glycine amidinotransferase gene (GATM) located to chromosome 15q15.1. This gene encodes AGAT, which converts arginine and glycine to ornithine and guanidino-acetate in the creatine cycle pathway. AGAT deficiency is transmitted in an autosomal recessive manner and genetic counseling is possible.