We compared the blood pressure (BP) measurements obtained with a random-zero sphygmomanometer and an ambulatory BP monitor in older persons with isolated systolic hypertension at one site of the multicenter, randomized, double-blind clinical trial, the Systolic Hypertension in the Elderly Program (SHEP) randomized clinical trial. The subjects were community-dwelling elderly participants with isolated systolic hypertension enrolled in the SHEP study and already receiving stable doses of double-blind medication (n = 35 for active treatment group; n = 32 for placebo group). We measured seated (clinic) BP obtained with a random-zero sphygmomamanometer, pulse rate, and BP and heart rate measurements obtained with an ambulatory BP monitor (average 24 h, daytime, and nighttime BP and heart rate). Across treatment groups clinic and ambulatory systolic BPs were not significantly different, but the placebo group had higher ambulatory, but not clinic, diastolic BPs. Within each treatment group (active treatment and placebo) there were no significant differences between clinic and average 24 h, daytime, or nighttime ambulatory systolic BPs. There were also no significant differences between clinic diastolic BP and average 24 h, daytime, or nighttime ambulatory diastolic BPs in the active treatment group, but in the placebo group average 24 h diastolic BP obtained by the ambulatory monitor was 4.6 mm Hg higher than clinic diastolic BP (P = .001). The average 24 h heart rate was 6 to 7 beats/min higher as measured by the ambulatory monitor compared to clinic pulse (P < .01). In the placebo group of this study, average 24 h ambulatory diastolic BPs were consistently higher than clinic diastolic BPs.(ABSTRACT TRUNCATED AT 250 WORDS)