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Previous studies have shown a U- or J-shaped association of body mass index (BMI) or change in BMI with coronary heart disease (CHD) among middle-aged and elderly adults. However, whether a similar association exists among young adults is unclear.To determine whether an association exists between BMI or BMI change with CHD among young adults.This population-based longitudinal study used data obtained by the Korean National Health Insurance Service from 2002 to 2015. The study population comprised 2 611 450 men and women aged between 20 and 39 years who underwent 2 health examinations, the first between 2002 and 2003 and the second between 2004 and 2005.World Health Organization Western Pacific Region guideline BMI categories of underweight, normal weight, overweight, obese grade 1, and obese grade 2 derived during the first health examination and change in BMI calculated during the second health examination.Body mass index (calculated as weight in kilograms divided by height in meters squared). Absolute risks (ARs), adjusted hazard ratios (aHRs), and 95% CIs for acute myocardial infarction or CHD during follow-up from 2006 to 2015.Data from 1 802 408 men with a mean (SD) age of 35.1 (4.8) years and 809 042 women with a mean (SD) age of 32.5 (6.3) years were included. The mean (SD) BMI was 23.2 (3.2) for the total population, 24.0 (3.0) for men, and 21.4 (2.9) for women. Compared with normal weight men, overweight (AR, 1.38%; aHR, 1.18 [95% CI, 1.14-1.22]), obese grade 1 (AR, 1.86%; aHR, 1.45 [95% CI, 1.41-1.50]), and obese grade 2 (AR, 2.69%; aHR, 1.97 [95% CI, 1.86-2.08]) men had an increased risk of CHD (P < .001 for trend). Similarly, compared with normal weight women, overweight (AR, 0.77%; aHR, 1.34 [95% CI, 1.24-1.46]), obese grade 1 (AR, 0.95%; aHR, 1.52 [95% CI, 1.39-1.66]), and obese grade 2 (AR, 1.01%; aHR, 1.64 [95% CI, 1.34-2.01]) women had an increased risk of CHD (P < .001 for trend). Compared with participants who maintained their weight at normal levels, those who became obese had elevated CHD risk among men (0.35% increase in AR; aHR, 1.35 [95% CI, 1.17-1.55]) and women (0.13% increase in AR; aHR, 1.31 [95% CI, 0.95-1.82]). Weight loss to normal levels among obese participants was associated with reduced CHD risk for men (0.58% decrease in AR; aHR, 0.77 [95% CI, 0.64-0.94]) and women (0.57% decrease in AR; aHR, 0.66 [95% CI, 0.45-0.98]).Obesity and weight gain were associated with elevated risk of CHD among young adults in this study. Studies that prospectively determine the association between weight change and CHD risk are needed to validate these findings.