18 adults with nonnephrotic proteinuria showed a milder form of idiopathic membranous nephropathy on renal biopsy. These patients were allocated to two groups, 2 years' steroid treatment or nontreatment. 8 of 9 tested patients showed complete disappearance of proteinuria in an average time of 2.4 months after starting the treatment, but in 9 patients of the nontreatment group proteinuria continued for at least 1 year. All five repeated biopsy specimens obtained after subsidence of proteinuria in the treatment group revealed a distinctive regression of glomerular basement membrane alterations. Though the study was not a controlled one, these results suggest that steroid therapy in idiopathic membranous nephropathy is beneficial, particularly in patients with lesser degrees of basement membrane lesions.