Background: Intrinsically disordered proteins and protein regions (IDPs/IDRs) are important in diverse biological processes. Lacking a stable secondary structure, they display an ensemble of conformations. One factor contributing to this conformational heterogeneity is the proline cis/trans isomerization. The knowledge and value of a given cis/trans proline ratio are paramount, as the different conformational states can be responsible for different biological functions. Nuclear Magnetic Resonance (NMR) spectroscopy is the only method to characterize the two co-existing isomers on an atomic level, and only a few works report on these data. Methods: After collecting the available experimental literature findings, we conducted a statistical analysis regarding the influence of the neighboring amino acid types (i \pm 4 regions) on forming a cis-Pro isomer. Based on this, several regularities were formulated. NMR spectroscopy was then used to define the cis-Pro content on model peptides and desired point mutations. Results: Analysis of NMR spectra prove the dependence of the cis-Pro content on the type of the neighboring amino acid—with special attention on aromatic and positively charged sidechains. Conclusions: Our results may benefit the design of protein regions with a given cis-Pro content, and contribute to a better understanding of the roles and functions of IDPs.