This study aimed to evaluate the diversity of clinical trials informing assessments conducted by the Institute for Clinical and Economic Review (ICER). This was a cross-sectional study of pivotal trials included in completed ICER assessments over 5 years (2017 –2021). Representation of racial/ethnic minority groups, females, and older adults was compared to the disease-specific- and US-population, using a relative representation cut-off of 0.8 for adequate representation. A total of 208 trials evaluating 112 interventions for 31 unique conditions, were examined. Race/ethnicity data were inconsistently reported. The median participant-to-disease representative ratio (PDRR) for Blacks/African Americans (0.43 [IQR:0.24, 0.75]), American Indians/Alaska Natives (0.37 [IQR:0.09, 0.77]), and Hispanics/Latinos (0.79 [IQR:0.30,1.22]) were below the adequate representation cut-off. In contrast, Whites (1.06 [IQR:0.92,1.2]), Asians (1.71; [IQR: 0.50, 3.75]), and Native Hawaiian/Other Pacific Islanders (1.61; [IQR:0.77,2.81]) were adequately represented. Findings were similar when compared to the US census, except for Native Hawaiian/Pacific Islanders, which was substantially worse. Relative to all trials, a higher proportion of US-based trials adequately represented Blacks/African Americans (61% vs. 23%, p<0.0001) and Hispanics/Latinos (68% vs. 50%; p=0.047), but a lower proportion adequately represented Asians (15% vs. 67%, p<0.0001). Females were adequately represented in 74% of trials (PDRR:1.02 [IQR: 0.79,1.14]). However, older adults were adequately represented in only 20% of trials (PDRR: 0.30 [IQR: 0.13,0.64]). The representation of racial/ethnic minorities and older adults was inadequate. Efforts are needed to enhance the diversity of clinical trials. Standardized and transparent evaluation of trial diversity should be part of the HTA process.