A review is presented of 22 published cases of verified or probable mitochondrial 2-methylacetoacetyl-CoA thiolase deficiency, a disorder of isoleucine and ketone body metabolism. The clinical expression, characterized by ketoacidosis, vomiting and lethargy, is highly variable. Typical age of onset is between 6 and 24 months. The disorder, which has been observed in several ethnic groups, is apparently inherited as an autosomal, recessive trait. The prognosis is relatively good if acute episodes of ketoacidosis and dehydration are adequately treated. There is abnormal urinary excretion of 2-methyl-3-hydroxybutyric acid, tiglylglycine, and in some instances also 2-methyl-acetoacetic acid. However, such a pattern of organic aciduria has also been found in cases with normal thiolase activity. Genetic complementation analyses have demonstrated considerable heterogeneity. The cDNA for human methyl-acetoacetyl-CoA thiolase has been cloned and sequenced. Studies in one patient showed a G-to-A substitution at position 1138 of the mRNA, causing 347Ala to Thr change in the mature enzyme. Studies in other patients have shown variable enzyme amount and/or stability.