Kim JH, et al. - In this study involving premenopausal underweight Korean women, researchers studied the prevalence and trends of breast cancer-related risk factors and characteristics according to birth year cohort. A total of 13,415 premenopausal women were investigated for determination of socioeconomic and breast cancer-related risk factors. Body mass index < 18.5 kg/m² defined underweight status. Multivariable models were also used. Of the overall sample, 9.5% consisted of underweight women. An increased risk of breast cancer was observed in relation to distinctive features, except for a lower rate of early menarche, observed in underweight Korean premenopausal women. These associations were prominent in recent generations.