The natural course of idiopathic membranous nephropathy is variable, with some patients slowly progressing to renal failure while others maintain normal renal function over the entire time. Whether to treat this disease or not is controversial due to the lack of controlled data about the long-term effects of treatment. We updated at 10 years the results of a controlled trial in which 81 patients with idiopathic membraneous nephropathy and nephrotic syndrome were randomly assigned to receive symptomatic therapy (39 patients) or a treatment of six months with methylprednisolone and chlorambucil (42 patients). The probability of surviving without developing end-stage renal disease at 10 years was 92% in patients given methylprednisolone and chlorambucil versus 60% in controls (P = 0.0038). The slope of the reciprocal of plasma creatinine up to 10 years was significantly better in treated patients than in controls (P = 0.035). The probability of having a complete or partial remission of the nephrotic syndrome was significantly higher in treated patients (P = 0.000). Patients assigned to therapy spent significantly longer time without nephrotic syndrome than untreated patients (P = 0.0001). Four patients had to stop treatment because of reversible side-effects. In the long-term one treated patient developed diabetes and another one became obese. In conclusion, a six-month therapy with methylprednisolone and chlorambucil increases the probability of remission of proteinuria and protects from renal function deterioration even in the long-term. This treatment may avoid dialysis or death within 10 years to about one third of nephrotic patients with membranous nephropathy.