Higher concentration of serum C4 level (HR1.059, 95% CI [1.021, 1.098], p = 0.002) and higher intensity of glomerular C4c deposits (HR1.505, 95% CI [1.061, 2.135], p = 0.022) predicted unfavorable renal outcome, as did age (HR0.969, 95% CI [0.943–0.996], p = 0.024), diabetic neuropathy (HR 0.117, 95% CI [0.015, 0.888], p = 0.038), serum C3 levels (HR 1.024, 95% CI [1.007,1.043], p = 0.007), serum creatinine (HR 1.007, 95% CI [1.003-1.011], p = 0.001) and 24 h urinary protein (HR 1.161, 95% CI [1.096–1.230], p < 0.001) in DKD patients. Here, C4A is linked to diabetic kidney disease.