3-Methylglutaconic aciduria has been described in two distinct syndromes. In one there was deficient 3-methylglutaconyl coenzyme A hydratase in fibroblast extracts where the only clinical manifestation was retarded speech development. In the second syndrome, the enzyme activity was normal but prominent neurological deterioration was noted. We describe two siblings with 3-methylglutaconic aciduria with normal enzyme activity who had choreoathetoid movements, optic atrophy, and mild developmental delay. The boy demonstrated developmental improvement in his second year of life, and his sister developed well, with normal school performance. These patients represent a new clinical variant of the second syndrome with a relatively favorable prognosis.

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