Background and objectives: Crescent formation are frequently observed in renal biopsies from IgA nephropathy (IgAN) patients. The clinical effect and therapeutic efficacy of a low percentage of glomeruli showing crescents still remains debated in IgAN except rapidly progressive cases. Therefore, we hypothesize that CF negatively affects proteinuria and shows an unfavorable response of proteinuria to renin angiotensin aldosterone system (RAAS) blockade, mainstay of IgAN treatment. Method: We retrospectively assessed the effects of CF on basal proteinuria at biopsy and on 1-year time-averaged proteinuria (TA-PU) under RAAS blockade without any immunosuppressant. Subjects were 281 patients (133 men, 148 women) with biopsy-proven primary IgAN from Seoul National University Bundang Hospital in Korea. Results: The mean estimated glomerular filtration rate and basal proteinuria values were 86.8 ml/min/1.73m2 and 1.67 g/g, respectively. CF was observed in 26.3% of patients. The presence of CF were associated with higher basal proteinuria in adjusted analyses including other histologic features (r=0.37, 95% CI 0.08 to 0.1, p<0.001). In 167 patients treated by RAAS blockade without any immunosuppressant, CF was associated with higher 1-year TA-PU in adjusted analyses (r=0.19, 95% CI 0.02 to 0.07, p=0.001). Conclusions: Crescents, histopathologic lesion, are associated with higher basal proteinuria, and the presence of CF blunts the anti-proteinuric effect of RAAS blockade in patients with IgAN. These finding suggested that early use of immunosuppressants including corticosteroid could be considered in IgAN patients showing crescents.