PURPOSE: Bladder outlet obstruction (BOO) causes storage and voiding dysfunction in the lower urinary tract. We investigated the expression of transient receptor potential cation channel subfamily M member 8 (TRPM8) to evaluate the relationship between TRPM8 expression and overactive bladder (OAB) in a rat model of BOO. METHODS: Fifty female Sprague-Dawley rats were divided into 4 groups; normal (n=10), normal-menthol (n=10), BOO (n=15), BOO-menthol (n=15). After 3 weeks, cystometry was performed by infusing physiological saline and menthol (3 mM) into the bladder at a slow infusion rate. The histological changes and expression of TRPM8 in the bladder were investigated by Masson's trichrome staining, immunofluorescence and reverse transcription-polymerase chain reaction. RESULTS: Cystometry showed that the intercontraction interval (ICI; 428.2+/-23.4 vs. 880.4+/-51.2, P<0.001), micturition pressure (MP; 25.7+/-1.01 vs. 71.80+/-3.01, P<0.001), and threshold pressure (2.9+/-0.25 vs. 9.2+/-1.58, P<0.01) were significantly increased in BOO rats. The bladder wall was significantly dilated compared with the control. Detrusor muscle hypertrophy and a thick mucosa layer were observed in BOO bladder. After menthol treatment, ICIs were decreased and MPs were increased in the menthol treatment groups. TRPM8-positive cells and mRNA were predominantly increased in the bladder and dorsal root ganglia of all groups compared with the normal group. CONCLUSIONS: Increased bladder wall thickness and proportion of collagen probably affect voiding dysfunction. Furthermore, an increase of TRPM8 expression in BOO may induce entry of Ca2+ from the extracellular space or stores. The increase of Ca2+ probably causes contraction of smooth muscle in BOO. However, OAB symptoms were not observed after menthol treatment although the expression of TRPM8 was abundant in the bladder epithelium after menthol treatment. Although OAB in BOO models may be caused by complex pathways, regulation of TRPM8 presents possibilities for OAB treatment.