We studied the relationship between the effectiveness of corticosteroid pulse treatment, clinical characteristics and outcomes in 17 patients with acute exacerbatin of idiopathic interstitial pneumonia. Acute exacerbation was defined as a decrease in PaO2 by more than 10 Torr, worsening of chest-radiographic findings, and deterioration of symptoms within 1 month. Effects of corticosteroid pulse treatment were judged 2 weeks and 3 months after the end of treatment. The change in AaD2 was computed and was used to determine whether the treatment had been effective, temporarily effective, or ineffective. The effects at three months were difficult to predict on the basis of those at two weeks. The treatment was often effective in patients who had not previously received steroids and in those with mild disease. Those in whom the treatment was effective at three months lived longer after the exacerbation than did those in whom it was ineffective, but the average lengths of survival through the whole clinical course were not significantly different among the three groups. We suggest that corticosteroid pulse treatment is effective in acute exacerbatins of idiopathic interstitial pneumonia and that effectiveness should be evaluated three months after treatment.