Chronic pain affects 20 % of the global population, causes suffering, is difficult to treat, and constitutes a large economic burden for society. So far, the characterization of molecular mechanisms of chronic pain-like behaviors in animal models has not translated into effective treatments. In this thesis, consisting of five studies, pain patient biofluids were analyzed with modern proteomic methods to identify biomarker candidates that can be used to improve our understanding of the pathophysiology chronic pain and lead to more effective treatments. Paper I is a proof of concept study, where a multiplex solid phase-proximity ligation assay (SP-PLA) was applied to cerebrospinal fluid (CSF) for the first time. CSF reference protein levels and four biomarker candidates for ALS were presented. The investigated proteins were not altered by spinal cord stimulation (SCS) treatment for neuropathic pain. In Paper II, patient CSF was explored by dimethyl and label-free mass spectrometric (MS) proteomic methods. Twelve proteins, known for their roles in neuroprotection, nociceptive signaling, immune regulation, and synaptic plasticity, were identified to be associated with SCS treatment of neuropathic pain. In Paper III, proximity extension assay (PEA) was used to analyze levels of 92 proteins in serum from patients one year after painful disc herniation. Patients with residual pain had significantly higher serum levels of 41 inflammatory proteins. In Paper IV, levels of 55 proteins were analyzed by a 100-plex antibody suspension bead array (ASBA) in CSF samples from two neuropathic pain patient cohorts, one cohort of fibromyalgia patients and two control cohorts. CSF protein profiles consisting of levels of apolipoprotein C1, ectonucleotide pyrophosphatase/phosphodiesterase family member 2, angiotensinogen, prostaglandin-H2 D-isomerase, neurexin-1, superoxide dismutases 1 and 3 were found to be associated with neuropathic pain and fibromyalgia. In Paper V, higher CSF levels of five chemokines and LAPTGF-beta-1were detected in two patient cohorts with neuropathic pain compared with healthy controls. In conclusion, we demonstrate that combining MS proteomic and multiplex antibody-based methods for analysis of patient biofluid samples is a viable approach for discovery of biomarker candidates for the pathophysiology and treatment of chronic pain. Several biomarker candidates possibly reflecting systemic inflammation, lipid metabolism, and neuroinflammation in different pain conditions were identified for further investigation. / Uppsala Berzelii Technology Centre for Neurodiagnostics