To highlight creatine deficiency syndromes (CDS) as rare but potentially partially treatable causes of common early neurologic presentations by describing clinical and metabolic characteristics associated with guanidinoacetate methyltransferase (GAMT) deficiency in a family. CDS are inborn errors of creatine metabolism manifesting with developmental delay, epilepsy, movement disorder and behavioural problems. Basic laboratory results, such as low plasma creatinine, can suggest these disorders leading to further metabolic testing, although low-normal “routine” levels may mislead. Oral creatine substitution can improve epilepsy, movement disorder and behaviour, but not intellectual ability, in some patients with GAMT deficiency. A case report suggests presymptomatic treatment may prevent disease manifestations. Two sisters, children of third cousins, were diagnosed in their early twenties after numerous previous negative investigations for other disorders. Both had severely delayed developmental motor and cognitive milestones, symptomatic epilepsy and dystonia, severe and generalised in the older sister. Low plasma creatinine led to further testing and GAMT deficiency was identified. Our case highlights the importance of intermittently re-investigating undiagnosed patients and considering metabolic diagnoses in patients with developmental delay, epilepsy and movement disorder. Early recognition is essential as appropriate therapy may improve certain clinical features.