Lupus nephritis (LN) class II has generally been considered a mild form of LN with a good response to treatment. Although the number was small, there have also been reports on later progression to class III or IV, resulting in poor renal and patient outcome. This study aims to review cases of LN class II to analyze differences between cases that progressed to class III or IV and cases that did not. We retrospectively analyzed 15 cases of LN class II among 277 cases of biopsy-proven lupus nephritis diagnosed in a tertiary medical center over about 14 years. Among the 15 patients, 5 patients progressed to class III or IV. Biopsy specimens were reviewed by a pathologist according to the ISN/RPS 2003 classification. Response to treatment was evaluated at 6 months after treatment. On fluorescence microscopy (IF), there was significantly higher degree of deposition in the glomeruli of IgM, IgA and C4 in the progression group than in the non-progression group. At 6 months after treatment, there was a trend toward higher rates of complete remission in the non-progression group (90%) compared with those in the progression group (40%, p = 0.077). Five of the 15 cases of ISN/RPS 2003 class II glomerulonephritis progressed to class III or IV over a mean of 5 years. The degree of immune-complex deposition for IgM, IgA and C4 in the glomeruli was significantly higher in the progression group.