A systematic review of randomized clinical trials (RCTs) in idiopathic inflammatory myopathies (IIMs) from 2010 to 2023 examined the reporting and representation of sex, race, ethnicity, and geographic distribution. Of the 19 RCTs reviewed, 58% appropriately reported race, while only 26% reported ethnicity, and 16% conflated the two. Black, Asian, and Hispanic or Latino participants accounted for just 3%, 7%, and 2% of enrollees, respectively. Encouragingly, most trials (90%) included over 45% women. However, there were no enrolling sites in South America or Africa, and most U.S. sites were located in large metropolitan areas, with none in nonmetropolitan regions and only 31% in medically underserved areas. These findings highlight significant gaps in the sociodemographic and geographic diversity of IIM RCTs. The underreporting of race and ethnicity and limited representation across populations and regions raise concerns about the generalizability and equity of clinical trial findings.