Six patients with interstitial lung disease were tested in a serial fashion to determine response to medications. All patients had biopsy-proven interstitial lung disease. Exercise testing with measurements of PaO2, PaCO2, and DLCO (the diffusing capacity to carbon monoxide) were made before and after exer cise to determine the response to medications. Three patients were treated with corticosteroids and three patients with cy totoxic medications. Results of serial testing of gas transfer correlated with im provement of interstitial lung disease as characterized by resting Pa02, symptoms, and maximal duration of exercise. The value of serial exercise test ing is of great help in some, but not all patients. The value of serial testing of patients with interstitial lung disease is discussed.