The most common clinical syndromes associated with 3-methylglutaconic aciduria are presented. In some patients these syndromes are multisystemic, progressive disorders of unknown etiology. Tissues deriving significant energy through oxidative metabolism (notably brain and cardiac muscle) are most often affected and in some the primary defect may reside within the mitochondrial respiratory chain. Although increasing biochemical evidence suggests that 3-methylglutaconic aciduria may correlate with deranged mitochondrial energy metabolism, the biochemical origin of 3-methylglutaconic acid and the significance of its increased excretion remain unknown. This review describes these syndromes and illustrates the necessity of urinary organic acid analysis to assist in the differential diagnosis.