HSD10 mitochondrial disease (HSD10MD), originally described as a deficiency of 2‐methyl‐3‐hydroxybutyryl‐CoA dehydrogenase (MHBD), is a rare X‐linked disorder of a moonlighting protein encoded by the HSD17B10. The diagnosis is usually first suspected on finding elevated isoleucine degradation metabolites in urine, reflecting decreased MHBD activity. Here, HSD17B10 is linked to inborn mitochondrial metabolism disorder.