This investigation examined cross-sectional and longitudinal relations, both linear and nonlinear, of blood pressure (BP) and its interaction with demographic and lifestyle variables to a broad spectrum of cognitive functions. Eight hundred forty-seven participants (503 men and 344 women) from the Baltimore Longitudinal Study of Aging completed tests of verbal and nonverbal memory, attention, perceptuo-motor speed, executive functions, and confrontation naming, and clinical assessment of BP on 1 to 7 occasions over 11 years. Mixed-effects regression models, adjusted for age, education, gender, alcohol consumption, smoking status, depression scores, and use of antihypertensive medications, revealed nonlinear relations of systolic BP with longitudinal change on tests of nonverbal memory and confrontation naming; cognitive decline was apparent among older (80 years) individuals with higher systolic BP. Cross-sectional findings, across testing sessions, indicated moderated U- and J-shaped relations between BP and cognitive function. Both high and low diastolic BP were associated with poorer performance on tests of executive function and confrontation naming among less-educated persons; with tests of perceptuo-motor speed and confrontation naming among nonmedicated (antihypertensives) individuals; and with executive function among older individuals. Cross-sectional linear relations included higher systolic BP and poorer nonverbal memory in nondrinkers, and higher diastolic BP and poorer working memory among less-educated individuals. Results indicate that cross-sectional and longitudinal relations of BP to cognitive function are predominantly nonlinear and moderated by age, education, and antihypertensive medications. Careful monitoring and treatment of both high and low BP levels may be critical to the preservation of cognitive function.