Twenty-four-hour recordings of intraarterial blood pressure (IABP) from 723 untreated hypertensive patients were analyzed for the effects of age, sex, race, and body mass index on the level of IABP and its circadian variation. Age had a highly significant positive relationship (P < .001) with the cuff systolic and diastolic blood pressures, with regression coefficients (SE) of +0.83 (0.07) and -0.24 (0.04) mm Hg/year, respectively. There was a similar (P < .001) positive relationship between age and 24-h mean systolic IABP, measuring +0.71 (0.07) mm Hg/year, but 24-h mean diastolic IABP did not increase significantly with age. There was a significant (P < .001) inverse relationship between age and 24-h mean heart rate (HR), at -0.17 (0.03) beats/min/year. Nocturnal fall in systolic and diastolic IABP, calculated as the difference between daytime and nighttime mean IABP, had a significant (P < .001) negative relationship with age. Nocturnal fall in HR, calculated similarly, also significantly (P < .001) decreased with age. Age did not affect long-term systolic and diastolic IABP variability but did decrease long-term HR variability significantly (P < .001). Hypertensive men and women of similar age, had comparable daytime mean systolic and diastolic IABP (P = .15 and P = .03 respectively), but women had significantly (P < .001) lower nighttime mean systolic and diastolic IABP than men. The nocturnal fall in systolic and diastolic IABP was significantly (P < .002) greater in women as compared to men. Women also had significantly (P < .01) greater long-term systolic and diastolic IABP variability than men. Women had significantly (P < .001) greater 24-h, daytime mean and nighttime mean HR than men. Twenty-four-hour, daytime and nighttime mean IABP were all significantly higher (P < .01) in Afro-Caribbeans as compared to whites and Asians. No significant differences were observed in the magnitude of nocturnal IABP fall or long-term IABP variability between the three races. Asians and Afro-Caribbeans had significantly (P < .001) lower nocturnal HR falls and long-term HR variability (P < .01) than whites. Body mass index (BMI) did not relate directly to the level of daytime blood pressure, clinic cuff, or daytime mean IABP, in either men or women. BMI did have a highly significant (P < .001) positive relationship with nighttime mean IABP in men, but not in women. The degree of nocturnal fall of IABP had a significant (P < .001) inverse relationship with BMI in hypertensive men.