The association between membranous nephropathy (MN) and cancer is often mentioned in textbooks but poorly substantiated, and the characteristics of cancer-associated MN are unknown. To address these questions, we studied a cohort of 240 patients with MN, among them 24 had malignancy at the time of renal biopsy or within a year thereafter. The incidence of cancer was significantly higher in these patients than in the general population (standardized incidence ratio 9.8 [5.5-16.2] for men and 12.3 [4.5-26.9] for women). The frequency of malignancy increased with age. At the time of diagnosis, clinical presentation did not differ between the patients with cancer-associated MN and those with idiopathic MN, but smoking was more frequent among patients with cancer. Analysis of renal biopsies revealed that the number of inflammatory cells infiltrating the glomeruli was significantly higher in patients with cancer-associated MN (P = 0.001). The best cutoff value for distinguishing malignancy-related cases from controls was eight cells per glomerulus. Using this threshold led to a diagnosis of cancer-associated MN with a specificity of 75% and a sensitivity of 92%. In patients with cancer-associated MN, there was a strong relationship between reduction of proteinuria and clinical remission of cancer (P < 0.001). In conclusion, our study provides epidemiologic evidence of an excess of cancer risk in patients with MN. It also shows that age, smoking, and the presence of glomerular leukocytic infiltrates strongly increase the likelihood of malignancy in MN patients.