The advancement of global health equality is being hindered by persistent disparities in women's health data. Despite comprising over half of the global population, women remain underrepresented or poorly characterized in many health datasets. According to the Gender Data Outlook (GDO) Index, developed by PARIS21 and UN Women, while progress has been made, systemic barriers continue to obstruct comprehensive gender data collection and dissemination. The index shows that even in high-income countries like Japan and the United Kingdom, strong data production exists, but institutional gaps and coordination issues persist. This issue is further exacerbated in low- and middle-income countries (LMICs).Key areas such as menstrual and reproductive health, aging, and gender-based differences in both non-communicable and communicable diseases are often overlooked or insufficiently disaggregated by sex, gender, age, ethnicity, and geography (Figure 1). This lack of inclusive data leads to diagnostic delays, ineffective treatments, and policies that fail to reflect women’s realities. Bias is further intensified by the underrepresentation of intersectional factors such as gender identity, socio-economic status, and disability, making health systems less responsive to women’s needs, especially in underserved populations.To address these gaps, investments in gender-sensitive, community-driven data systems, equitable research funding, and ethical data governance are essential. Improving women's health data is not only crucial for research but is also a matter of justice, critical to achieving the Sustainable Development Goals (SDGs). Currently, countries lack 44% of the data needed to track 51 gender-related SDG indicators, including SDG-5 on gender equality. Without urgent action, global health initiatives risk exacerbating disparities rather than improving them. As the 2030 deadline for the SDGs approaches rapidly, it is our responsibility to take the necessary actions to close gender data gaps.