Aims: Bladder pain syndrome/interstitial cystitis (BPS/IC), diagnosed according to the new 2008 criteria of the European Society for the Study of Interstitial Cystitis (ESSIC), may lead to detrusor fibrosis. In some inflammatory diseases, fibrosis is related to YKL-40. The aims were to examine YKL-40 antigenic expression in bladder tissue and levels in serum and urine in BPS/IC and to evaluate whether YKL-40 could be a non-invasive, prognostic biomarker for bladder fibrogenesis and treatment intensity. Methods and results: Immunohistochemistry, immunoelectron microscopy and enzyme-linked immunosorbent assay (ELISA) analyses in 45 patients showed YKL-40 expression in detrusor mast cell granules and submucosal macrophages, and elevated YKL-40 levels in serum and urine compared to healthy individuals (median 72 versus 7 mu g/l, P <0.001). Clinicopathological parameters showed associations of detrusor fibrosis with YKL-40-positive cells (P = 0.001), mast cells (P = 0.014) and urine YKL-40 (P = 0.009). Bladder capacity correlated inversely with YKL-40-positive cells (P <0.001) and mast cells (P = 0.029). Treatment intensity was not associated with YKL-40. Conclusion: Serum and urine levels of YKL-40 may be used as non-invasive biomarkers in BPS/IC for the evaluation of bladder fibrogenesis