Objective: Obesity contributes directly to the risk of diabetes and hypertension. Effective management of diabetes is essential to prevent or delay the onset of comorbid hypertension. In this study, we delineate the association body mass index (BMI) has with risk and age at onset of hypertension and explore how this association is modulated by sex and the pre-existing condition of diabetes. Design: Cross-sectional study using retrospective data. Setting: Kuwait Health Network that integrates primary health and hospital laboratory data into a single system. Participants: We considered 3904 native Kuwaiti comorbid individuals who had the onset of type 2 diabetes prior to that of hypertension, and 1403 native Kuwaiti hypertensive individuals with no incidence of diabetes. These participants have been regularly monitored for BMI, glycated haemoglobin and blood pressure measurements. Mean variance in BMI per individual over the period from registration is seen to be low. Main outcome measures: Association between age at onset of hypertension and BMI (as measured at hypertension diagnosis); HRs for developing hypertension. Results: Risk of hypertension increases with obesity levels, and is higher in patients with diabetes than in patients without diabetes but of similar obesity levels. Age at onset of hypertension is inversely related to BMI; this relationship is seen to be stronger in men compared to women (slope estimate in men,-0.62 years per unit increase in BMI; in women-0.18) and in individuals (particularly women) with diabetes compared to those without (slope estimate in women,-0.39 vs-0.18, p<0.001; in men-0.66 vs-0.62; p=0.66). Conclusions: The observation that the presence of diabetes doubles the slope of inverse relationship between hypertension onset age and BMI in women (while the slope is high in men irrespective of diabetes status) leads to a possible proposition that pre-existing diabetes narrows down sex-specific differences in the impact of obesity on blood pressure.