Background and Aims: The evolution of renal disease and its global impairment in patients diagnosed with ANCA-associated vasculitis (AAV) has been related to baseline C3 serum levels in several studies. The objective of this study is to evaluate in patients diagnosed with AAV and renal impairment the association between baseline serum C3 levels and the clinical outcomes. Method: Our cohort included 56 patients diagnosed with AAV diagnosed from January 1986 to January 2018 in Hospital La Paz. According to C3 serum level measurements, patients were divided into low and normal concentration groups at onset. Histological, clinical and laboratory data were compared between groups. Results: Among the 56 AAV cases, 58.9% were women and 96.5% Caucasian with an age mean and SD of 61.08 ± 15.11 years. Mean follow-up was 10.03 ± 7.02 years. MPO ANCAs were found in 69.8% of the patients and PR3 ANCAs in 22.64%. AAV diagnosis was earlier in PR3 AVV (47.59 ± 13.62 years) compared to MPO AVV (65.66 ± 11.53 years) (p=0.001). At diagnosis, baseline serum C3 was measured in 53 patients (94.6%) being lower in MPO AAV (112.79 ± 33.98) vs PR3 AAV (132.41 ± 20.69) (p=0.023). Low C3 was way more frequent in patients with higher percentage of glomeruli showing >50% of sclerosis (100% vs 32.5%) (p=0.007). Staphylococcus aureus nasal carriers were more frequent in patients with low C3 (27.2% vs 3.1%, p=0.045). 15 patients (27.77%) died during follow-up and patient survival median was 21.75 years (IC 95%: 15.41-28.08). 17 patients (31.48%) reached end-stage renal disease (ESRD) during follow-up and renal survival median was 20.1 years (IC 95%: 19.27-21.05). There were no statistical differences in the levels of baseline serum C3 according to sex, relapses, extrarenal manifestations, treatment, malignancy, cardiovascular events and time to ESRD or death. Conclusion: Baseline serum C3 was lower in MPO AV, in Staphylococcus aureus nasal carriers and higher severity histological damage (glomerular sclerosis >50%). Despite previous evidence, in our cohort, time to ESRD and time to death seemed not to be affected.