The crystal structure of 3alpha-hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni (3alpha-HSDH) as well as the structure of its binary complex with NAD(+) have been solved at 1.68-A and 1.95-A resolution, respectively. The enzyme is a member of the short chain dehydrogenase/reductase (SDR) family. Accordingly, the active center and the conformation of the bound nucleotide cofactor closely resemble those of other SDRs. The crystal structure reveals one homodimer per asymmetric unit representing the physiologically active unity. Dimerization takes place via an interface essentially built-up by helix alphaG and strand betaG of each subunit. So far this type of intermolecular contact has exclusively been observed in homotetrameric SDRs but never in the structure of a homodimeric SDR. The formation of a tetramer is blocked in 3alpha-HSDH by the presence of a predominantly alpha-helical subdomain which is missing in all other SDRs of known structure.