Some patients with chronic idiopathic interstitial pneumonia (IIP) experience acute exacerbations (AE). Because the precise mechanisms of AE in patients with IIP remain unclear, the treatment for AE is not established and the efficacy of steroids is controversial. Consequently, it is difficult to predict outcomes in patients with AE of IIP. We therefore studied the relationship between clinical findings, efficacy of treatment, and clinical outcome in patients with AE of IIP. Thirty-two patients were enrolled, and were divided into two groups: 10 who were alive more than one year after the onset of the AE survivors, 8 men and 2 women, and 22 who died within one year of the AE (non-survivors, 17 men and 5 women). Survivors were significantly younger than non-survivors (59.7 +/- 9.9 vs 67.5 +/- 8.2 years, respectively, p < 0.05). The values of PaCO2 measured before the AE were higher in survivors than in non-survivors (43.0 +/- 3.7 vs 38.4 +/- 4.0 torr, respectively, p < 0.05). At the onset of the AE the levels of C-reactive protein in serum were higher in survivors than in non-survivors (13.9 +/- 7.9 vs 7.3 +/- 5.8 mg/dl, respectively, p < 0.05). Chest X-ray films showed progression of ground-glass shadows in both groups when the AE occurred; the radiographic findings did not differ markedly between groups. Of the 22 non-survivors, 7 had received medication before the AE; none of the survivors had received medication before the AE. At the time of the AE all patients were treated with steroid pulse therapy, and the dose of methylprednisolone used, did not differ significantly between groups. These data suggest that three factors are closely related to responsiveness to steroid therapy and to clinical outcomes after AE in patients with IIP: 1) age at the onset on IIP, 2) respiratory status before the AE, and 3) disease activity as reflected by inflammatory reactions.