BackgroundRisk factors for rare gynecological cancers are largely unknown. Initial research has indicated that the metabolic syndrome (MetS) or individual components could play a role. Materials and methodsThe Metabolic syndrome and Cancer project cohort includes 288834 women. During an average follow-up of 11 years, 82 vulvar, 26 vaginal and 43 other rare gynecological cancers were identified. Hazard ratios (HRs) were estimated fitting Cox proportional hazards regression models for tertiles and standardized z-scores [with a mean of 0 and a standard deviation (SD) of 1] of body mass index (BMI), blood pressure, glucose, cholesterol, triglycerides and MetS. Risk estimates were corrected for random error in the measurement of metabolic factors. ResultsThe MetS was associated with increased risk of vulvar [HR 1.78, 95% confidence interval (CI) 1.30–2.41) and vaginal cancer (HR 1.87, 95% CI 1.07–3.25). Among separate MetS components, 1 SD increase in BMI was associated with overall risk (HR 1.43, 95% CI 1.23–1.66), vulvar (HR 1.36, 95% CI 1.11–1.69) and vaginal cancer (HR 1.79, 95% CI 1.30–2.46). Blood glucose and triglyceride concentrations were associated with increased risk of vulvar cancer (HR 1.98, 95% CI 1.10–3.58 and HR 2.09, 95% CI 1.39–3.15, respectively). ConclusionThe results from this first prospective study on rare gynecological cancers suggest that the MetS and its individual components may play a role in the development of these tumors.