Human type 10 17beta-hydroxysteroid dehydrogenase (HSD) is a homotetrameric protein located in mitochondria. This enzyme was alternatively named short chain L-3-hydroxyacyl-CoA dehydrogenase (SCHSD). This NAD(H)-dependent dehydrogenase is essential for the metabolism of branched-chain fatty acids and isoleucine, and is expressed in a variety of tissues, e.g., prostate, brain, liver, and heart. This enzyme inactivates 17beta-estradiol and exhibits a strong oxidative 3alpha-HSD activity to convert 5alpha-androstanediol and allopregnanolone into 5alpha-dihydrotestosterone (5alpha-DHT) and 5alpha-dihydroprogesterone, respectively, in living cells. Certain malignant prostatic epithelial cells and activated astrocytes in Alzheimer's disease patient's brain contain extraordinarily high levels of this enzyme. This mitochondrial dehydrogenase enables prostate cancer cells to generate 5alpha-DHT in the absence of testosterone. Its inactivation of allopregnanolone is important to the modulation of GABA(A) receptor. Among steroidogenic enzymes 17beta-HSD10 plays a significant part in the intracrinology. Although this protein has an affinity for amyloid-beta peptide, its role in the pathogenesis of Alzheimer's disease is far from clear. Additional knowledge of this versatile enzyme would provide the foundation for designing new drugs aimed at treating some neurological diseases and certain types of cancers.