Since the first description of a creatine deficiency syndrome, the guanidinoacetate methyltransferase (GAMT) deficiency, in 1994, the two further suspected creatine deficiency syndromes--the creatine transporter (CrT1) defect and the arginine:glycine amidinotransferase (AGAT) deficiency were disclosed. GAMT and AGAT deficiency have autosomal-recessive traits, whereas the CrT1 defect is a X-linked disorder. All patients reveal developmental delay/regression, mental retardation, and severe disturbance of their expressive and cognitive speech. The common feature of all creatine deficiency syndromes is the severe depletion of creatine/phosphocreatine in the brain. Only the GAMT deficiency is in addition characterized by accumulation of guanidinoacetic acid in brain and body fluids. Guanidinoacetic acid seems to be responsible for intractable seizures and the movement disorder, both exclusively found in GAMT deficiency. Treatment with oral creatine supplementation is in part successful in GAMT and AGAT deficiency, whereas in CrT1 defect it is not able to replenish creatine in the brain. Treatment of combined arginine restriction and ornithine substitution in GAMT deficiency is capable to decrease guanidinoacetic acid permanently and improves the clinical outcome. The lack of the creatine/phosphocreatine signal in the patient's brain by means of in vivo proton magnetic resonance spectroscopy is the common finding and the diagnostic clue in all three diseases. In AGAT deficiency guanidinoacetic acid is decreased, whereas creatine in blood was found to be normal. On the other hand the CrT1 defect is characterized by an increased concentration of creatine in blood and urine whereas guanidinoacetic acid concentration is normal. The increasing number of patients detected very recently suffering from a creatine deficiency syndrome and the unfavorable outcome highlights the need of further attempts in early recognition of affected individuals and in optimizing its treatment. The study of creatine deficiency syndromes and their comparative consideration contributes to the better understanding of the pathophysiological role of creatine and other guanidino compounds in man.