Summary: The significance of tubulointerstitial fibrosis in paediatric patients with primary diffuse mesangial proliferative glomerulonephritis and IgM deposition (IgM nephropathy) has not been well documented. the clinical course, therapeutic response and final outcome of 35 patients in whom renal biopsies showed IgM nephropathy are reported. They have been subdivided into two groups according to the absence (19 patients: group I) or presence (16 patients: group II) of superimposed lesion of tubulointerstitial fibrosis. Clinical presentation was similar in both groups but the patients in group I were male predominant (2.8:1 vs 1: 1). 6/19 (31.6%) of the patients in group I and 1/16 (6.3%) in group II responded to corticosteroid, 11/19 (57.9%) in group I and 8/16 (50.0%) in group II had steroid dependent, whereas 2/19 (10.5%) in group I and 7/16 (43.7%) in group II had steroid resistant. About 42% of the patients in group I and 94% in group II required cyclophosphamide therapy by which similar response in both groups (approximately 75%) were observed. the steroid non-responders and cyclophosphosphamide therapy among patients in group II were significantly higher (P<0.05) than the group I. At the latest assessment, 5/16 (31.3%) in group I and 7/14 (50%) in group II had impaired renal function. the follow-up period in both groups were 3.1 ± 2.8 and 3.4 ± 2.9 years, respectively. In conclusion, the finding of tubulointerstitial fibrosis in a paediatric IgM nephropathy indicates an unfavourable therapeutic response