In 56 patients with borderline hypertension, casual blood pressure was compared with average daytime ambulatory blood pressure. The difference between these two values ranged from -20 to +40 mm Hg and was unrelated to spontaneous blood pressure variability, the blood pressure fall during sleep, and the peaks of pressure recorded during the day. Heart rate and a family history of hypertension were not predictive of the degree of difference between casual and average daytime recorder pressure. Target organ damage was present in 44% of the patients and was more closely correlated with recorder than with casual pressure.