The effects of childhood obesity or weight gain on the development of early-onset follicular occlusion triad (FOT) among children, which includes hidradenitis suppurativa (HS), acne conglobata (AC), and dissecting cellulitis of the scalp (DCS), remain unknown. To investigate the association between body mass index (BMI) or its changes and early-onset FOT development. This nationwide population-based longitudinal cohort study included a cohort of 2,012,928 Korean children who underwent two consecutive health examinations at 30-36 months and 42-48 months of age between 2009 and 2020. The BMI and its changes derived during health screenings at 30-36 and 42-48 months. We performed multivariate Cox proportional hazards regression analyses to estimate the risk of early-onset FOT, including HS, AC, and DCS. During the follow-up, 1,283 FOT, including 143 HS, 1,068 AC, and 72 DCS, events were identified. Children with obesity were at an elevated risk of early-onset FOT compared to those with a normal weight (FOT: adjusted hazard ratio [aHR] 1.49, 95% confidence interval [CI] 1.21-1.84; HS: aHR 2.30, 95% CI 1.39-3.82; AC: aHR 1.36, 95% CI 1.07-1.73). BMI gain was correlated with an elevated risk of early-onset FOT, particularly HS, whereas BMI loss was linked to a decreased risk of early-onset FOT, especially HS. Children who became obese had an increased early-onset FOT risk (aHR 1.51, 95% CI 1.07-2.14) compared to those who maintained a normal weight. Children who reduced from obese to normal weight exhibited a decreased risk of early-onset FOT (FOT: aHR 0.41, 95% CI 0.17-0.96, P for trend=0.02; HS: P for trend=0.05) compared to obese children who retained their weight. Childhood obesity was associated with an elevated risk of early-onset FOT, including HS and AC. Weight gain was correlated with an increased risk of early-onset FOT, especially HS, while weight loss was associated with a decreased risk of early-onset FOT, particularly HS. Implementing purposeful weight-reduction strategies may be helpful in preventing early-onset FOT development.