Abstract Context Breast development is important for most trans women. An important limitation of current breast development measurement methods is that these do not allow for 3D volume analyses. Objectives To examine breast development and change in anthropometry during the first 3 years of gender-affirming hormone treatment using 3D imaging. Associations with clinical or laboratory parameters and satisfaction with the gained breast development were also studied. Design Prospective cohort study. Setting Specialized tertiary gender identity clinic in Amsterdam, the Netherlands. Participants Participants were 69 adult trans women with a median age of 26 years (interquartile range, 21-38). Interventions Gender-affirming hormone treatment. Main Outcome Measures Volumetric and anthropometric breast development and satisfaction. Results Breast volume increased by 72 cc (95% confidence interval [CI], 48-97) to 100 cc (standard deviation 48). This resulted in a cup-size <A-cup in 71% of the participants. Although the change in breast-chest difference plateaued after approximately 9 months, sustained increase in breast volume was observed during the 3-year observation period. Sternal notch to nipple distance increased by 1.3 cm (95% CI, 0.9-1.7) and internipple distance increased by 1.0 cm (95% CI, 0.4-1.5). At least 58% of trans women were satisfied with the gained breast size. Conclusions Sustained breast growth and development during hormone treatment was observed during the full 3-year observation period. The breasts of trans women are positioned more laterally and caudally on the chest compared with cis women. Although modest breast volumes were observed, breast development was satisfactory to most trans women.