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Recently, 3 patients with a creatine synthesis defect have been described. They presented with developmental regression, extrapyramidal movement abnormalities, and intractable epilepsy, and they improved with treatment of creatine monohydrate. We report 2 unrelated boys with a creatine synthesis defect and nonspecific presenting signs of psychomotor retardation, behavioral problems, and, in 1, mild epilepsy. Metabolic urine screening revealed elevations in all metabolites, expressed as millimoles per mole of creatinine, which suggests decreased creatinine excretion. This finding led to the correct diagnosis. We propose to include the assessment of the overall concentrations of amino acids and organic acids relative to creatinine in routine metabolic urine screening. Ann Neurol 2000;47:540–543.