3-hydroxy-3-methylglutaric acid is a dicarboxylic acid that is glutaric acid in which one of the two hydrogens at position 3 is substituted by a hydroxy group, while the other is substituted by a methyl group. It has been found to accumulate in urine of patients suffering from HMG-CoA lyase (3-hydroxy-3-methylglutaryl-CoA lyase, EC 4.1.3.4) deficiency. It occurs as a plant metabolite in Crotalaria dura. It has a role as an antimetabolite, an EC 1.1.1.34/EC 1.1.1.