To provide a therapeutic agent and a diagnostic agent for mitochondrial dysfunction brain diseases including cerebral infarction as a representative. A therapeutic agent for a mitochondrial dysfunction brain disease, which comprises (A) .delta.-aminolevulinic acid represented by the formula (1), a derivative thereof, or a salt thereof and (B) an iron compound in combination. R.sup.2R.sup.1NCH.sub.2COCH.sub.2CH.sub.2COR.sup.3 (1) wherein R.sup.1 and R.sup.2 each independently represents a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group, or an aralkyl group; and R.sup.3 represents a hydroxy group, an alkoxy group, an acyloxy group, an alkoxycarbonyloxy group, an aryloxy group, an aralkyloxy group, or an amino group.