Background and Purpose As risk of hemorrhagic stroke may have early life origins, we investigated associations of birth weight and childhood body mass index (BMI) with adult intracerebral hemorrhage (ICH) or subarachnoid hemorrhage (SAH). Methods We included 240,234 Danish schoolchildren, born 1936 to 1989, with information on birth weight and measured weights and heights from 7 to 13 years. We calculated hazard ratios (HRs) and confidence intervals (CIs) for the associations between early life anthropometrics and ICH or SAH, identified through linkage with national registers. Results During the study period, 1,947 individuals (39% women) experienced an ICH and 797 individuals (64% women) experienced a SAH. Per 500 g increase in birth weight, women had a 10% decreased risk of SAH (HR, 0.90; 95% CI, 0.83 to 0.97) and men had a 10% decreased risk of ICH (HR, 0.90; 95% CI, 0.85 to 0.95). Birth weight was not associated with risks of ICH in women or SAH in men. In men, a childhood BMI below average (BMI z-score <0) was associated with increased risks of ICH. The association was stronger at older childhood ages, and at 13 years a BMI z-score of -1 was associated with a HR of 1.17 (95% CI, 1.06 to 1.28), and a BMI z-score of -2 with a HR of 1.46 (95% CI, 1.17 to 1.82) for ICH. Childhood BMI was not associated with risks of ICH in women or with risks of SAH in both sexes. Conclusions Early life body size is associated with ICH and SAH, and the associations differ by sex.