Background: ω–3 polyunsaturated acids therapy is efficient in primary IgA nephropathy. It is unknown whether doses of ω–3 smaller than those given previously are still effective. The aim of the study was to examine the effect of ω–3 therapy on renal vascular function in relation to proteinuria and urinary excretion of N-acetyl-β- D-glucosaminidase (NAG). Methods: 20 IgA patients aged 36.5 ± 10.77 with creatinine clearance (Cr<sub>cl</sub>) 105.71 ± 27.3 ml/min and proteinuria 3.31 ± 2.01 g/24 h were given orally 810 mg EPA and 540 mg DHA daily for 12 months. Before and at the end of the study, 24-hour proteinuria, serum homocysteine, and Cr<sub>cl</sub> were measured. At the same time, renal vascular function was estimated as dopamine-induced glomerular filtration response (DIR). DIR was measured as: two 120-min lasting Cr<sub>cl</sub> (before and during 2 µg/kg b.w./min i.v. dopamine). Results: The results obtained during follow-up were as follows (baseline vs. after therapy): DIR 14.9 ± 16.4 vs. 30.3 ± 14.3% (p < 0.01); urine protein 2.31 ± 2.01 vs. 1.31 ± 1.37 g/24 h (p < 0.01); (Cr<sub>cl</sub>) 105.71 ± 27.3 vs. 103.9 ± 20.9 ml/min (n.s.); NAG 8.3 ± 1.8 vs. 6.0 ± 1.2 U/g<sub>creat</sub> (p < 0.01), and homocysteine 16.2 ± 3.15 vs. 13.8 ± 2.6 µmol/l (p < 0.05). The only correlation found was linear correlation between basal DIR and DIR change (r = –0.570; p < 0.010) and basal NAG (r = –0.460; p < 0.50). Conclusions: ω–3 supplementation is associated with the improvement of both renal vascular function and tubule function.