BACKGROUND: Blood pressure is normally lower during night times than in day times. But in some people, this nocturnal blood pressure drop is decreased or absent. In essential hypertensives, absence of nocturnal blood pressure drop (the 'nondipping' pattern) has been associated with increased target organ damage and cardiovascular morbidity. Present study was intended to describe the proportion of 'nondippers' among untreated essential hypertensives in Korea and to investigate the influences of independent clinical factors on the nocturnal fall of blood pressure. METHODS: We studied 50 untreated Korean essential hypertensives who underwent 24-hour ambulatory blood pressure monitoring. Subjects with a nocturnal drop in systolic or diastolic blood pressure, or both, 10% were defined as 'dippers', the others as 'nondippers'. Their demographic and clinical characteristics were obtained through medical record and questionnaire. Above characteristics were considered to be independent variables and the dipping status, dependent. RESULTS: Thirty four percent of essential hypertensives were 'nondippers'. Nocturnal bloGd pressure drop decreased with reporting of family history of hypertension and higher 24-hour ambulatory mean b1ood pressure. Previous studies insisted on the influence of age on the dipping status, but in our study, age showed no impact on nocturnal blood pressure drop. CONCLUSION: About one third of untreated subjects with essential hypertension were observed to be nondippers. Family history of hypertension and high 24-hour ambulatory mean blood pressure significantly decreased nocturnal blood pressure drop in our study. According to the previous studies, above factors might work through affecting the normally decreased sympathetic activity during sleep time. Longitudinal studies of the influence of nondipping on the long term prognosis of hypertension and large scale studies to establish the factors influencing nocturnal blood pressure drop and their mechanisms are further needed.