Corticosteroids have had variable success in IgA nephropathy (IgAN). Our previous trial with a six-month course of steroids in IgAN patients showed they were effective in reducing the risk of renal function deterioration and proteinuria, but this effect seemed to decrease in the long term. This new randomised trial was designed to prospectively evaluate whether adding low-dose azathioprine to steroids improves long-term renal survival in adult biopsy-proven IgAN patients with proteinuria > or = 1 g/24 h and plasma creatinine < or = 2.0 mg/dl. The patients will be treated with steroids (methylprednisolone 1 g i.v. for three consecutive days at months 1, 3 and 5, plus oral prednisone 0.5 mg/kg every other day for six months) plus azathioprine 1.5 mg/kg/day for six months or steroids alone with the same schedule. Altogether a minimum of 346 patients should be enrolled within a four-year recruitment period. The planned duration of follow-up is five years.