Purpose: This study describes participant diversity in Williams syndrome (WS) intervention studies. Methods: A literature search was conducted to identify prospective treatment studies including participants with WS. Data was extracted on the reporting of and information provided on age, sex, cognitive ability, socioeconomic status, race, and ethnicity. Results: Eleven eligible articles were identified. Reporting rates of demographic factors varied considerably, with the highest rates for age and sex (100%) and the lowest reporting rates for race (18%) and ethnicity (9%). Combining demographic data from the two studies that reported on race and/or ethnicity (n = 33), 88% of participants were White. The combined participant mean age was 20.9 years. Conclusion: There is a low frequency of reporting on several demographic factors including socioeconomic status, race, and ethnicity in WS intervention studies. There is a need for increased representation of racial and ethnic minority groups, older participants, and more cognitively impaired patients in WS research.