Calreticulin (CRT) is an abundant molecular chaperone of the endoplasmic reticulum. Its central, proline-rich P-domain, comprising residues 189-288, contains three copies of each of two repeat sequences (types 1 and 2), which are arranged in a characteristic '111222' pattern. Here we show that the three-dimensional structure of CRT(189-288) contains a single hairpin fold formed by the entire polypeptide chain. The loop at the bottom of the hairpin consists of residues 227-247, and is closed by an anti-parallel beta-sheet of residues 224-226 and 248-250. Two additional beta-sheets contain residues 207-209 and 262-264, and 190-192 and 276-278. The 17-residue spacing of the beta-strands in the N-terminal part of the hairpin and the 14-residue spacing in the C-terminal part reflect the length of the type 1 and type 2 sequence repeats. As a consequence of this topology the peptide segments separating the beta-strands in the N-terminal part of the hairpin are likely to form bulges to accommodate the extra residues. These results are based on nearly complete sequence-specific NMR assignments for CRT(189-288), which were obtained using standard NMR techniques with the (13)C/(15)N-labeled protein, and collection of nuclear Overhauser enhancement upper distance constraints.