The purpose of this retrospective study was to study the incidence of idiopathic and secondary forms of membranous nephropathy in our institution, its clinical course and progression to chronic renal failure, and the risk factors associated with it. Two hundred fourteen (16%) of the 1,287 renal biopsies obtained between 1962 and 1988 were primary glomerular diseases and 28 of this 214 (13%) were idiopathic membranous nephropathy. On the other hand 59 of 1,287 biopsies were membranous nephropathy of whom 28 were idiopathic, 27 secondary to systemic lupus erythematosus, 2 due to drugs, one associated with rheumatoid arthritis, and one more with breast cancer. The clinical picture was: nephrotic syndrome in 84%, hypertension in 15%, non-nephrotic proteinuria in 14%, chronic renal failure in 8.4%, and renal vein thrombosis in 6.3%. In the idiopathic group 75% of the patients were male while in the lupus group 85% were female. For the analysis of progression to chronic renal failure we excluded 5 patients with renal failure when the biopsy was taken, 2 because the nephropathy was due to drugs, one associated with breast cancer, and nine were within the first year of follow-up. Thus, for this analysis the group consisted of 22 patients with idiopathic form and 20 with systemic lupus erythematosus. The idiopathic and lupus groups were similar except for a lower serum albumin in the former. The progression to renal failure was seen in 9 patients: six in the idiopathic group and the other 3 in the lupus group; this difference was not significant.(ABSTRACT TRUNCATED AT 250 WORDS)