ImportanceRacial and ethnic disparities in delayed medical care for reasons that are not directly associated with the cost of care remain understudied.ObjectiveTo describe trends in racial and ethnic disparities in barriers to timely medical care among adults during a recent 20-year period.Design, Setting, and ParticipantsThis was a serial cross-sectional study of 590 603 noninstitutionalized adults in the US using data from the National Health Interview Survey from 1999 to 2018. Data analyses were performed from December 2021 through August 2022.ExposuresSelf-reported race, ethnicity, household income, and sex.Main Outcomes and MeasuresTemporal trends in disparities regarding 5 specific barriers to timely medical care: inability to get through by telephone, no appointment available soon enough, long waiting times, inconvenient office or clinic hours, and lack of transportation.ResultsThe study cohort comprised 590 603 adult respondents (mean [SE] age, 46.00 [0.07] years; 329 638 [51.9%] female; 27 447 [4.7%] Asian, 83 929 [11.8%] Black, 98 692 [13.8%] Hispanic/Latino, and 380 535 [69.7%] White). In 1999, the proportion of each race and ethnicity group reporting any of the 5 barriers to timely medical care was 7.3% among the Asian group; 6.9%, Black; 7.9%, Hispanic/Latino; and 7.0%, White (P > .05 for each difference compared with White individuals). From 1999 to 2018, this proportion increased across all 4 race and ethnicity groups (by 5.7, 8.0, 8.1, and 5.9 percentage points [pp] among Asian, Black, Hispanic/Latino, and White individuals, respectively; P < .001 for each), slightly increasing the disparities between groups. In 2018, compared with White individuals, the proportion reporting any barrier was 2.1 and 3.1 pp higher among Black and Hispanic/Latino individuals (P = .03 and P = .001, respectively). There was no significant difference in prevalence between Asian and White individuals. There was a significant increase in the difference in prevalence between Black individuals and White individuals who reported delaying care because of long waiting times at the clinic or medical office and because of a lack of transportation (1.5 pp and 1.8 pp; P = .03 and P = .01, respectively). In addition, the difference in prevalence between Hispanic/Latino and White individuals who reported delaying care because of long waiting times increased significantly (2.6 pp; P < .001).Conclusions and RelevanceThe findings of this serial cross-sectional study of data from the National Health Interview Survey suggest that barriers to timely medical care in the US increased for all population groups from 1999 to 2018, with associated increases in disparities among race and ethnicity groups. Interventions beyond those currently implemented are needed to improve access to medical care and to eliminate disparities among race and ethnicity groups.