Objectives: The aim of this study was to investigate the effects of infant and toddler head growth on intelligence scores from early childhood to adulthood in very preterm (<32 weeks gestational age; VP) and/or very low birth weight (<1500 g; VLBW) and term born individuals. Methods: 203 VP/VLBW and 198 term comparisons were studied from birth to adulthood as part of the prospective geographically defined Bavarian Longitudinal Study (BLS). Head circumference was assessed at birth; 5, 20 months; and 4 years of age. Intelligence was assessed with standardized tests in childhood (6 and 8 years: K-ABC) and at 26 years (Wechsler Adult Intelligence Scale, WAIS). Structural equation modeling (SEM) was used to model the effect of head growth on IQ. Results: On average, VP/VLBW had lower head circumference at birth (27.61 cm vs. 35.11 cm, mean difference 7.49, 95% confidence interval [7.09-7.90]) and lower adult intelligence scores (88.98 vs. 102.54, mean difference 13.56 [10.59-16.53]) than term born comparison individuals. Head circumference at birth (e.g., total effect β=.48; p<.001 for adult IQ) and head growth in childhood predicted intelligence development from age 6 to 26 years in both VP/VLBW and term born individuals (70% of variance in adult IQ explained by full model). Effects of gestation and birth weight on intelligence were fully mediated by head circumference and growth. Conclusions: This longitudinal investigation from birth to adulthood indicates head growth as a proxy of brain development and intelligence. Repeated early head circumference assessment adds valuable information when screening for long-term neurocognitive risk. (JINS, 2019, 25, 48#x2013;56)