Aims: Intelligence and education have both been associated with body mass index. However, few longitudinal studies have simultaneously investigated the importance of intelligence and education for changes in body mass index (BMI) over time. Methods: This longitudinal study included 1536 Danish men with information about intelligence, educational level and BMI in young adulthood and BMI 41 years later in late midlife. Results: Mean BMI increase was 5.1 kg/m². Higher intelligence in young adulthood was significantly associated with less gain in BMI. The association attenuated when adjusting for level of education. Higher education was associated with less gain in BMI and these associations remained significant when adjusting for intelligence. Conclusion: The findings suggest that both intelligence and educational level in young adulthood are associated with changes in BMI from young adulthood to late midlife with education being the stronger predictor of BMI changes across the adult life course.