We studied clinical and radiographic features of interstitial lung disease (ILD) during trimethoprim/sulfamethoxazole (TMP/SMX) administration. Ten patients who had received prednisolone treatment for underlying diffuse pulmonary disease showed various ILDs after introduction of TMP/SMX. The radiographic features of the ILDs were not consistent with infectious disease or exacerbation of the underlying disease, and these diagnoses were excluded radiographically and on clinical grounds during the differential diagnosis of the ILDs. These ILDs emerged relatively early after introduction of TMP/SMX, which is consistent with the former case report of drug-induced ILD (DI-ILD) caused by TMP/SMX. Therefore DI-ILDs caused by TMP/SMX were suspected in these cases. In most of these cases, the ILDs were clinically mild and disappeared immediately although administration of TMP/SMX was continued.