Background Psoriasis is a chronic and non-transient disease with increased epidermal proliferation in the skin. Dysregulation of the immune system is an important factor in this pathology. Inflammation markers, pro-inflammatory cytokines, and immune cells are reported to be changed in psoriasis. Study design In the current cohort study, the possible changes in interleukin-6 (IL-6), neopterin levels, and kynurenine (Kyn) pathway in 42 psoriasis patients compared to 30 controls, and their change with narrow-band (NB) UVB treatment were investigated. Methodology IL-6 and serum neopterin levels were analyzed with ELISA kits. HPLC analyses were performed to detect urinary neopterin, serum Kyn, and tryptophan (Trp) levels. Results IL-6 levels were lower, while Kyn levels and the Kyn-to-Trp ratio were higher in psoriasis patients compared to control subjects ( p < 0.01, all). Conclusion Narrow-band ultraviolet B (NB-UVB) treatment decreased Psoriasis Area and Severity Index (PASI) scores and increased urinary neopterin levels of the patients (both, p < 0.01). Serum neopterin was correlated with Kyn and Kyn/Trp levels before and after NB-UVB treatment ( p < 0.05, all). These findings point out that the measured parameters might be considered to support the PASI score in both diagnosis and prognosis of psoriasis rather than evaluating the severity of the disease.