Ovarian cancer is the fifth leading cause of cancer-related mortality in women. Nicotinamide N-methyltransferase (NNMT) is a metabolic enzyme and there is growing evidence to suggest that it plays an important role in cancer progression. This is the first study to examine the expression of NNMT in serous ovarian cystadenomas, serous borderline tumours, low grade serous carcinomas (LGSC) and high grade serous carcinomas (HGSC) and investigate the potential independent association of NNMT expression with survival. Tissue samples were analysed immunohistochemically for NNMT expression. The stromal NNMT score was significantly higher in HGSC compared to serous cystadenomas and serous borderline tumours (p <.001, p <.043, respectively). The mean stromal NNMT score of patients with HGSC was significantly higher than patients with LGSC (p =.043). Patients with low expression of NNMT had a significantly higher mean recurrence-free survival than patients with high expression (p =.036). NNMT may support tumour progression in ovarian cancer by promoting desmoplastic stromal tumour reaction. NNMT overexpression may be associated with poor prognosis and can be a therapeutic target in ovarian cancer.IMPACT STATEMENTWhat is already known on this subject? Nicotinamide N-methyltransferase (NNMT) is a cytosolic enzyme that is overexpressed in many malignancies. Its overexpression was shown to lead to histone hypomethylation, which in turn can decrease and increase the expression of tumour suppressor proteins and onco-proteins, respectively. NNMT was also shown to play a role in epithelial-to-mesenchymal transition, which is critical in tumour progression and the stromal tumour reaction. The stromal tumour reaction was recently targeted with promising therapeutic results in ovarian cancer. What do the results of this study add? The expression of NNMT in various ovarian neoplasms including serous cystadenomas, borderline tumours and serous carcinomas has not been studied and independently associated with poor survival, previously. This study suggests that NNMT is progressively overexpressed in the stroma of ovarian neoplasms from benign cysts to HGSCs. NNMT overexpression appears to be independently associated with poor survival in ovarian cancer. What are the implications of these findings for clinical practice and/or further research? The implications of these findings are that NNMT may play an important role in the stromal tumour reaction, and therefore its overexpression may contribute to poor survival. NNMT overexpression may be an important target of ovarian cancer therapy.