Nonspecific interstital pneumonia (NSIP) is associated with an excellent response to steroidal therapy. However, it was reported that some cases of NSIP might have an unfavorable outcome in spite of aggressive therapy. In this study, we explored the prognosis for 23 patients with NSIP and examined matched groups of 18 patients with treatment-responsive or naturally recovered idiopathic NSIP, and 5 patients with idiopathic NSIP who fared poorly despite therapy. The patients who had poor prognoses were, significantly, all non-smoking women. Histologic examination revealed that the cases with progressive, idiopathic NSIP were in all cases of the fibrotic type. By contrast, only 39% of those with a good prognosis, were fibrotic. No significant differences in CT appearance were noted between the two cohorts. These findings provide a potential clinical marker of therapy outcome of in idiopathic NSIP.