The aim of this study is to report the long-term outcome of pure membranous lupus nephropathy (MLN) treated with glucocorticoid and azathioprine (AZA). A cohort of patients with SLE who had biopsy-confirmed pure MLN was treated initially with prednisone (0.8-1.0 mg/kg/day) and AZA (targeted to 2 mg/kg/day). Patients were followed for the following outcomes: remission rate at 12 months, renal flares, extra-renal flares and renal function deterioration. The cumulative risks of renal flares and renal function decline were studied by Kaplan-Meier analysis. Thirty-eight patients were studied (31 women; age 35.0 +/- 9.2 years; mean SLE duration 48.5 +/- 59 months; WHO Class Va 45%, Vb 55%). Twenty-two (58%) patients were nephrotic and four (11%) were hypertensive at presentation. All patients were treated with prednisolone (0.85 +/- 0.24 mg/kg/day) and AZA (1.72 +/- 0.43 mg/kg/day). At 12 months, 24 (67%) patients achieved complete response (CR), 8 (22%) had partial response (PR) and 4 (11%) were treatment resistant. After a follow-up of 12 +/- 5.8 years, 19 episodes of renal flares (15 proteinuric and 4 nephritic) occurred in 13 (34%) patients. The cumulative risks of renal flares at 5, 10 and 15 years were 19.4, 32.0 and 36.8%, respectively. Retreatment with an augmented dosage of prednisolone, +/- another immunosuppressive agent, resulted in CR and PR in 15 (79%) and 4 (21%) of these flare episodes, respectively. At last visit, three (8%) patients had doubling of serum creatinine, whereas six (16%) patients had decline of creatinine clearance by >/=30% (none developed end stage renal failure). Seven episodes of thromboembolic complications occurred in five (13%) patients and 11 episodes of infective complications (five major, six minor) were reported in seven (18%) patients. In the absence of co-existing proliferative lesions, MLN runs a relatively benign course with low risk of renal function deterioration. Treatment with high-dose prednisolone and AZA is effective, inexpensive and well-tolerated. Prolonged observation shows that one of three patients develop renal flares, which are often proteinuric and responsive to reinduction therapy.