Left ventricular (LV) hypertrophy is a major risk factor for cardiovascular morbidity. This study examined the extent to which exaggerated blood pressure (BP) reactivity is associated with LV mass and relative wall thickness (RWT) in a large sample of middle-aged hypertensive men and women. Participants included 1,012 African-Americans and 655 Caucasians recruited for the Hypertension Genetics Epidemiology Network (HyperGEN) study. A laboratory stress protocol was implemented, consisting of BP measurement during rest, handgrip, and a mental arithmetic stressor. This was followed by a 2-dimensional guided M-mode echocardiographic evaluation to measure LV mass. LV mass was greater among African-Americans relative to Caucasians (p <0.01). Systolic BP at rest was positively associated with LV mass and RWT in African-Americans (p <0.001) and in Caucasian women (p <0.0001). LV mass and RWT were compared in participants and participants were classified as high or low reactors based on their systolic BP responses to the 2 laboratory stressors. RWT demonstrated positive associations with systolic BP responses to both tasks. Associations with LV mass were mostly nonsignificant with the exception of a negative association in African-Americans between systolic BP responses to the arithmetic task and LV mass. We conclude that in this cohort of hypertensive middle-aged men and women, BP reactivity did not predict LV mass. Positive associations of systolic BP responses with RWT suggest that exaggerated reactivity may particularly be related to LV changes influenced by vascular tonus. © 2002 by Excerpta Medica, Inc.