The 3(17)alpha-hydroxysteroid dehydrogenase from mouse is involved in the metabolism of oestrogens, androgens, neurosteroids and xenobiotic compounds. The enzyme was crystallized by the hanging-drop vapour-diffusion method in space group P222(1), with unit-cell parameters a = 84.91, b = 84.90, c = 95.83 A. The Matthews coefficient (VM) and the solvent content were 2.21 A3 Da(-1) and 44.6%, respectively, assuming the presence of two molecules in the asymmetric unit. Diffraction data were collected to a resolution of 1.8 A at the Swiss Light Source beamline X06SA using a MAR CCD area detector and gave a data set with an overall Rmerge of 6.8% and a completeness of 91.1%.