In hypertensive patients, diminished nocturnal blood pressure (BP) fall is associated with poor prognosis for cardiovascular events. However, the relation of insulin resistance with the etiology of nondipper essential hypertension remains unclear. The aim of the present study was to assess the role of insulin resistance in diminished nocturnal BP fall, left ventricular hypertrophy (LVH), and increased plasma atrial (ANP) and brain natriuretic peptides (BNP) in essential hypertensive patients. One hundred and three patients with essential hypertension were divided into dippers (n =57; age: 57±5 years, mean±SD) or age-matched nondippers (n =46; 57±4 years), based on ambulatory BP (ABP) monitoring. Although the systolic and diastolic ABP values were similar during the day, those at night were higher in nondippers than in dippers (p <0.0001 for each). Echocardiographic findings revealed that the left ventricular mass index (LVMI) was higher in nondippers (p <0.0001). Plasma ANP and BNP were also higher in nondippers (p <0.0001 for each). Fasting plasma concentrations of glucose and insulin (p <0.0001 for each) and the homeostasis model assessment (HOMA) index (p <0.0001) were also higher in nondippers. Multivariate analysis revealed that systolic ABP at night was a significant factor for LVMI, ANP and BNP. In addition, the HOMA index was a significant factor for LVMI and BNP. These observations suggest that diminished nocturnal BP fall is closely related to the development of LVH with concomitant increase in BNP in essential hypertensive patients, and that insulin resistance may play a key role in these processes. (Hypertens Res 2003; 26: 669-676)