The first crystal structure of an oligoproline adopting an all-trans polyproline II (PPII) helix is presented. The high-resolution structure provides detailed insight into the dimensions and conformational properties of oligoprolines that are important for, e.g., their use as molecular rulers and molecular scaffolds. The structure also showed that the amides interact with each other within a PPII helix and that water is not necessary for PPII helicity.