We examined the relationship between acorn mass and number in valley oaks (Quercus lobata) over 4 years in central coastal California. Despite considerable variation in acorn size among both trees and years, trees produced acorns of the same size relative to other trees in different years. Across years, the relationship between acorn mass and acorn crop size was generally positive, even after controlling for environmental conditions and differences in individual tree size and quality. Life-history trade-offs in valley oaks are primarily between current and future reproduction and indirectly between concurrent growth and reproduction, not between seed size and number, and are probably related to this species' mast-seeding behavior. Phenotypic trade-offs in long-lived plants such as oaks exhibit complex patterns of life-history covariation and deserve greater attention, both theoretically and empirically.