Human type 10 17β -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 17β -estradiol and exhibits a strong oxidative 3α-HSD activity to convert 5α-androstanediol and allopregnanolone into 5α-dihydrotestosterone (5α-DHT) and 5α-dihydroprogesterone, respectively, in living cells. Certain malignant prostatic epithelial cells and activated astrocytes in Alzheimers disease patients brain contain extraordinarily high levels of this enzyme. This mitochondrial dehydrogenase enables prostate cancer cells to generate 5α-DHT in the absence of testosterone. Its inactivation of allopregnanolone is important to the modulation of GABA(A) receptor. Among steroidogenic enzymes 17β-HSD10 plays a significant part in the intracrinology. Although this protein has an affinity for amyloid-βpeptide, its role in the pathogenesis of Alzheimers 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.