This study aimed to provide foundational data for developing upper clothing patterns suitable for women with large breasts, defined as a bust–underbust difference of 15.0 cm or more (C-cup or larger). The study investigated wearing conditions, purchasing behavior, clothing preferences, and areas of dissatisfaction. Results showed that many women were unaware of their exact bust size and preferred slightly loose upper garments for daily wear to reduce discomfort. Online platforms were the main purchasing channels, with participants emphasizing adequate sizing, accessibility, and stretchable fabrics. Satisfaction with bras was consistently low, indicating insufficient support and functionality for women with large breasts. Despite their bust size, participants generally chose moderately loose rather than overly loose garments, often purchasing larger sizes than their actual measurements to compensate for poor fit. Common complaints included gaping front buttons and rising hems, especially in blouses and shirts. Raising the arms in blouses or jackets caused discomfort due to gaping, while women wearing larger T-shirt sizes (105 and above) reported greater fit issues such as back pulling, shoulder twisting, and elbow tightness. These findings suggest that simply enlarging sizes does not resolve fit problems. Limited design options for large-busted women highlighted a lack of pattern development, diversity, and design consideration. Key purchase criteria included proper bust fit, sufficient coverage, and prevention of front gaping. Based on these findings, the study proposes: (1) clothing patterns reflecting large-bust characteristics; (2) functional designs aligned with real wearing behavior; (3) expansion of design diversity; (4) material selection balancing elasticity, durability, and stability; and (5) specialized size systems and customized clothing. This research provides foundational data for improving the fit of ready-to-wear garments for women with large breasts but is limited to C-cup or larger wearers. Future research should include a wider range of bust sizes and body types to identify broader fit factors and advance inclusive clothing design.