The crystal structure of a mutant of bovine pancreatic trypsin inhibitor has been refined to 0.86 A resolution using low-temperature synchrotron data. The variant contains three mutations in the binding loop (Thr11Ala, Pro13Ala, Lys15Arg) and an unrelated Met52Leu substitution. Refinement with anisotropic displacement parameters and with removal of main-chain stereochemical restraints converged with R = 0.1035. The use of full-matrix refinement provided an estimate of the variances in the derived parameters. Some stereochemical parameters, such as the planarity of the peptide group and the value of the N-C(alpha)-C angle, show a wide spread, suggesting that the standard values used as restraints in protein structure refinements may not always be entirely appropriate. Comparison with the recently determined room-temperature structure of the same mutant at 1.42 A resolution confirms the previous observations and provides new details, such as a double conformation of the main chain at Leu29 and at Gly56-Gly57, a high proportion (over 20%) of residues in double conformations, correlation of disorder through lattice contacts and the positions of H atoms, including those in water molecules, and their involvement in C-H...O and N-H...pi hydrogen bonds.