ObjectiveQuantify differential attainment by ethnicity in undergraduate medical assessments and evaluate whether institutional efforts to reduce the attainment gap have had impact.DesignObservational cohort study.SettingA single UK MBBS medical programme.ParticipantsPseudonymised data of adults aged ≥18 years enrolled in one of the UK MBBS medical programmes between 2012 and 2018. Ethnicity was self-declared during enrolment as White, Asian, Black, mixed and other.Main outcome measureModule mark (distinction, merit, pass, fail) graded according to a variety of assessments, including single best answer examinations, objective structured clinical examinations and coursework submissions. All modular assessments are graded as a percentage. Logistic regression models were used to calculate relative risk ratio to study the association between ethnicity and attainment gap over a calendar and scholastic year. Models were adjusted for age, gender, social deprivation and scholastic year of study.Results3714 student records were included. In the sample, 2134 students (57%) were non-white. The proportion of non-white students increased from 2007 (49%) to 2018 (70%). Mean age was 18 (IQR 18–21) and 56.6% were females. Higher proportion of non-white students 218 (24.8%) were from more deprived backgrounds versus white 76 (14.8%). Compared with non-white, there were no significant differences in the proportion of students failing assessments. However, white students were more likely to achieve merit (relative risk ratio 1.29 (95% CI 1.08 to 1.45)) or distinction (1.69 (95% CI 1.37 to 2.08)). Differences in attainment gap have remained unchanged over time, and for black students, attainment gap grew between their first and final year of study.ConclusionA similar proportion (97%) of non-white and white students had a passing score, but attainment gap for higher grades persists over years despite widespread efforts in medical schools to diminish the attainment gap linked to ethnicity. Our findings are from a single institution, thus affecting generalisability.