3-Methylglutaconic aciduria (MGA) encompasses a heterogeneous group of disorders, often coinciding with elevated levels of urinary 3-methylglutaric acid. Type I MGA is a disorder of leucine metabolism, while the biological basis for the MGA is unclear for the other types (MGA types II-V). MGA type III (Costeff optic atrophy syndrome, autosomal recessive optic atrophy-3 or optic atrophy plus syndrome, OMIM 258501) is distinguished by early bilateral optic atrophy, later-onset spasticity, extrapyramidal dysfunction, ataxia, and occasional cognitive deficits. It is caused by homozygous mutations in the optic atrophy 3 gene (OPA3). We present a case of a patient with MGA who has infantile-onset optic atrophy, ataxia, extrapyramidal movements and spasticity, but with normal intellect. Sequencing of the patient's DNA revealed a homozygous nonsense mutation c.415C>T (p.Q139X) in exon 2 of transcript 2 of the OPA3 gene, as well as a common silent polymorphism c.231T>C in the same exon. This is the first nonsense mutation found in OPA3. The molecular findings in OPA3 are also reviewed, including mutations in OPA3 that result in autosomal dominant optic atrophy and cataract (ADOAC). The recessive mode of inheritance of MGA type III as a result of the p.Q139X mutation is supported by the carrier status of the unaffected father.