BACKGROUND: Policies for early detection of breast cancer, including clinical breast examinations and mammographic screening, were introduced in Brazil in 2004, but their effect on disease stage at diagnosis is unclear. We aimed to assess whether these policies have led to a decrease in the prevalence of late-stage breast cancer at diagnosis. METHODS: In this case only analysis, using an anonymised nationwide hospital based-cancer registry network, we identified women aged 18-89 years who had been diagnosed with an invasive breast cancer in Brazil during 2001-14. We extracted individual patient-level data on patient demographics, tumour variables, and health-care provider variables for the centre where the patient was diagnosed. Our objectives were to estimate the prevalence of late-stage breast cancer (TNM stage III or IV) at diagnosis overall, across age groups, and by ethnoracial and social strata (ie, self-reported ethnoracial group, as white, black, brown, Asian, or Indigenous, and educational level, marital status, and region of residence) across the study period, and compare these estimates with international data from high-income countries (Norway and the USA). We used logistic regression to estimate odds ratios (ORs) for late-stage versus early-stage (TNM stage I or II) breast cancer at diagnosis in relation to relevant exposures, either minimally adjusted (for age, year of diagnosis, and region of residence) or fully adjusted (for all patient, tumour, and health-care provider variables). FINDINGS: We identified 247 719 women who were diagnosed with invasive breast cancer between Jan 1, 2001, and Dec 31, 2014, with a mean age at diagnosis of 55·4 years (SD 13·3), of whom 36·2% (n=89 550) identified as white, 29·8% (n=73 826) as black or brown, and 0·7% (n=1639) as Asian or Indigenous. Prevalence of late-stage breast cancer at diagnosis remained high throughout 2001-14, at approximately 40%, was inversely associated with educational level (p value for linear trend <0·0001), and was higher for women who identified as black (minimally adjusted OR 1·61, 95% CI 1·53-1·70; fully adjusted OR 1·45, 95% CI 1·38-1·54) and brown (minimally adjusted OR 1·26, 95% CI 1·22-1·30; fully adjusted OR 1·18, 1·14-1·23) than those who identified as white. The predicted prevalence of late-stage cancer at diagnosis was highest for women who were black or brown with little or no formal education (48·8%, 95% CI 48·2-49·5) and lowest for women who were white with university education (29·4%, 28·2-30·6), but both these prevalences were higher than that of all women diagnosed with breast cancer in Norway before the introduction of mammography screening (ie, 16·3%, 95% CI 15·4%-17·2% in 1970-74). Similar ethnoracial and social patterns emerged in analyses restricted to the age group targeted by screening (50-69 years). INTERPRETATION: The persistently high prevalence of late-stage breast cancer at diagnosis across all ethnoracial and social strata in Brazil, although more substantially among the most disadvantaged populations, implies that early detection policies might have had little effect on breast cancer mortality so far, and highlights the need to focus primarily on timely diagnosis of symptomatic breast cancer rather than on screening for asymptomatic disease. FUNDING: Newton Fund, Research Councils UK, and Conselho Nacional das Fundações Estaduais de Amparo à Pesquisa.