Primary IgA nephritis is the most frequent nephritis and represent half of the glomerular diseases. Its incidence is 2,7/10(5) inhabitants. The diagnosis is established on a renal biopsy, which shows typical IgA mesangial deposits: granular, predominant, and diffuse. A central pathogenic role is played by the IgA immune system, both from the mucosa and the bone marrow. The long term prognosis should no longer be considered as mild because, after 20 years of evolution, 1/3 of the patients are going into chronic renal failure, and 1/6 into end-stage renal disease. It is essential to delineate the risk factors predicting ultimate evolution towards chronic failure, to select patients who may benefit from aggressive treatment, such as high doses steroids. For all patients, it is essential to have a regular clinical and biological check up, and an adequate control of arterial hypertension, in order to avoid or delay progression.