OBJECTIVES: Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a systemic vasculitis with unknown aetiology. Although biopsies are helpful for diagnosing AAV, whether they improve the outcomes of AAV patients remains unknown. The objective of this study was to elucidate the association between biopsies and prognosis. METHOD: This retrospective cohort study analysed health care records that were routinely collected at 190 hospitals in Japan from April 2005 to March 2019. Patients who were 18 years or older and hospitalized for AAV were eligible for inclusion. The primary outcome was the composite outcome of death and renal replacement therapy (RRT) during the first admission. We compared the outcome between the biopsy group and the no-biopsy group. The chi-square test was performed as a univariable analysis, and logistic regression analysis was performed as a multivariable analysis. RESULTS: Three hundred and eighty-six patients were assessed in this study. One hundred fifty-four (39.9%) patients underwent biopsy, and 232 (60.1%) patients did not undergo biopsy during the first admission with AAV. In univariable analysis, the composite outcome of death and RRT were observed in 7 (4.5%) patients in the biopsy group and 25 (10.8%) patients in the no-biopsy group (OR 0.39 [95% CI 0.17, 0.94], P = 0.01). The result was consistent in the multivariable analysis (OR 0.31 [95% CI 0.12, 0.79], P = 0.01) after adjusting for potential confounders. CONCLUSIONS: We showed that patients who underwent biopsy had a better prognosis in the composite outcome of mortality and RRT during admission using a Japanese healthcare record database. Key Points • This is the first study to elucidate the association between undergoing biopsy and prognosis. • Patients undergoing biopsy had a better prognosis in the composite outcome of mortality and RRT. • The proportion of patients who received transfusions was not significantly different between the two groups.