Recently, cyclosporin has been reported to be a promising drug for the treatment of interstitial pneumonia. Monitoring of the serum cyclosporin concentration is important for the safety and efficacy of treatment. We measured the concentrations of this drug just before (C0) and 2 hours after (C2) administration, and the area under the concentration-time curve from the start of administration for 5 hours (AUC 0-5) in 58 patients. We found that C2 has the strongest correlation with AUC 0-5, which indicates the efficacy of cyclosporin. In 11 cases of interstitial pneumonia, 5 showed deterioration despite cyclosporin treatment. Three of those 5 cases had low C2 and AUC 0-5 levels, indicating that they were low absorbers and slow absorbers, which may be associated with a poor response. Therefore, the monitoring of the cyclosporin concentration is important especially in progressive cases of interstitial pneumonia that deteriorate despite cyclosporin treatment.