IMPORTANCE: There is a lack of evidence on the association of birth weight, childhood body mass index (BMI), change in BMI during childhood, and childhood height with subsequent risks of hidradenitis suppurativa (HS) in adulthood. OBJECTIVE: To investigate the association of birth weight, childhood BMI, change in BMI during childhood, and childhood height with subsequent risks of HS in adulthood in a large Danish population-based cohort. DESIGN, SETTING, AND PARTICIPANTS: This cohort study included 347 200 schoolchildren from the Copenhagen School Health Records Register born from 1930 to 1996 who were linked to the Danish National Patient Register of hospital discharge diagnoses to identify cases of HS. Birth weight was reported by parents or guardians, whereas childhood weight and height were measured by school physicians or nurses at ages 7 through 13 years. Cox proportional hazards regressions were used to estimate hazard ratios (HRs) and 95% CIs. Statistical analysis was performed from February 20, 2019, to May 15, 2019. MAIN OUTCOMES AND MEASURES: A diagnosis of HS as recorded in the Danish National Patient Register. RESULTS: Among the 347 200 children included in the study (175 750 boys) during the follow-up period from 1977 to 2017, 1037 individuals (677 females; median age at diagnosis, 39 years [range, 15-73 years]) received a diagnosis of HS. A nonlinear (U-shaped) association was found between birth weight and HS, such that both the lightest (2.00-2.75 kg; HR, 1.36 [95% CI, 1.10-1.68]) and the heaviest babies (4.26-5.50 kg; HR, 1.39 [95% CI, 1.01-1.93]) had increased risks of HS compared with normal-weight babies (3.26-3.75 kg; P = .04 for deviation from linearity). The risk of HS increased significantly with increasing BMI z score at each age from 7 to 13 years, from an HR of 1.32 (95% CI, 1.24-1.40) per BMI z score at 7 years of age to an HR of 1.50 (95% CI, 1.40-1.61) per BMI z score at 13 years of age. Compared with children with a normal weight at 7 and 13 years of age, those with a normal weight at 7 years of age and overweight at 13 years of age had a significantly increased risk of HS (HR, 2.11 [95% CI, 1.63-2.74]) and children with persistent overweight at both ages also had an increased risk of HS (HR, 2.61 [95% CI, 2.02-3.38]). Children with overweight at 7 years of age but with normal weight at 13 years of age did not have a significantly increased risk of HS (HR, 1.05 [95% CI, 0.67-1.67]). Childhood height at all ages was not associated with risk of HS (children at 7 years had an HR of 1.00 [95% CI, 0.94-1.07], and those 13 years had an HR of 1.06 [95% CI, 0.99-1.13], per z score). CONCLUSIONS AND RELEVANCE: This cohort study found that both the lightest and heaviest babies had increased risks of HS. Childhood BMI was positively and significantly associated with risk of HS development in adulthood. These findings suggest that returning to normal weight before puberty reduces risks of HS to levels observed in children who were never overweight. Childhood height was not associated with risk of HS.