BACKGROUND The objective of this study is to determine whether initial steroid therapy is actually effective for the treatment of iMN, and we examined a 40% reduction in estimated glomerular filtration rate (eGFR) and remission rates. METHODS This was a retrospective study between 1993 and 2013. First, we divided patients with iMN having a urinary protein level of ≥1 g/gCre into two groups: those who had received steroid therapy (Group S1; n = 52) within 6 months of diagnosis and those who had received supportive therapy (Group H1; n = 31). Second, we compared 20 cases using propensity score matching (Group S2, Group H2). Third, we compared patients with a urinary protein level of 1-3.5 g/gCre (Group S3, n = 18; Group H3, n = 19) and those with a urinary protein level ≥3.5 g/gCre (Group S4, n = 34; Group H4, n = 12). The primary endpoint was a 40% reduction in eGFR, and the secondary endpoint was the achievement of complete remission (CR). RESULTS In Group S1 and Group H1, a 40% reduction in the eGFR was observed at the end of 5 years in 18 and 17% of the patients, respectively (P = 0.93); at the end of 10 years, these rates had increased to 43% and 50%, respectively (P = 0.88). The CR rates at the end of 5 years were 58% and 32%, respectively (P = 0.02), while the rates at 10 years were 65 and 39%, respectively (P = 0.02). No difference in renal outcomes was observed between Group S1 and Group H1. No significant differences were observed between Group S2 and Group H2, between Group S3 and Group H3, or between Group S4 and Group H4. CONCLUSIONS Initial steroid therapy is not superior to supportive care within the first 6 months after diagnosis in terms of a 40% reduction in eGFR.