Aromatase was purified from term human placenta by immuno-affinity chromatography in a highly active form. It was complexed with androstenedione and crystallized at 4 °C in the oxidized high-spin ferric state of the haem iron with poly(ethylene glycol) 4000 as the precipitant. The space group was P3221 and the unit cell parameters are a =b =140.2 Å, c =119.3 Å, α = β =90°, γ =120°, having one aromatase molecule in the asymmetric unit. Diffraction data at about 100 K were collected initially at the Cornell High Energy Synchrotron Source (CHESS) and then to 2.90 Å resolution at the Advanced Photon Source, Argonne National Laboratory, with glycerol as a cryoprotectant. Two data sets at the Fe absorption edge were also collected at the CHESS. The structure was solved by the molecular replacement method coupled with Bijvoet difference Fourier synthesis for identifying the correct solution. Model building and refinement were performed with Coot and Refmac5, respectively. The final model contained 452 amino acid residues; 44 N-terminal and 7 C-terminal residues could not be built because of weakness of their electron densities. The final R factor for all reflections between 38 and 2.90 Å resolution was 0.214, and the R-free value was 0.244. The r.m.s. deviations of bond lengths and angles from ideal values were 0.009 Å and 1.32°, respectively. The average isotropic thermal factor for all atoms was 77.3 Å2. There were only two violations in the backbone torsion angle Ramachandran plot, both in the loop regions. The oxyferryl Fe(IV)=O moiety was generated by adding an oxygen atom to Fe with the modelling software MOE (Chemical Computing Group) The exemestane molecule was built into the active site by superimposing it on the experimentally derived androstenedione atomic positions with MOE.