The purpose of this study was to investigate the long-term clinical course of non-specific interstitial pneumonia (NSIP) and to determine which factors are associated with a response to steroid therapy and relapse. Thirty-five patients with pathologically proven NSIP were included. Clinical, radiological, and laboratory data were reviewed retrospectively. The male-to-female ratio was 7:28 (median age, 52 yr). Thirty (86%) patients responded to steroid therapy, and the median follow-up was 55.2 months (range, 15.9-102.0 months). Five patients (14%) showed sustained disease progression and three died despite treatment. In the five with sustained disease progression, NSIP was associated with various systemic conditions, and the seropositivity of fluorescent antinuclear antibody was significantly associated with a poor response to steroids (P = 0.028). The rate of relapse was 25%, but all relapsed patients improved after re-treatment. The initial dose of steroids was significantly low in the relapse group (P = 0.020). In conclusion, progression is associated with various systemic conditions in patients who show progression. A low dose of initial steroids is significantly associated with relapse.