The prognosis for patients with liver metastases from colorectal carcinoma is limited because of the low number of patients who are eligible for curative hepatic resection. In this phase I study, 31 liver metastases in 24 patients with nonresectable metastases from colorectal carcinoma were treated with photodynamic therapy (PDT). The photosensitizer 5,10,15,20-tetrakis(m-hydroxyphenyl)bacteriochlorin (mTHPBC) was intravenously administered in a dose of .6 mg/kg (n = 12) or .3 mg/kg (n = 12). After 120 hours (n = 18) or 48 hours (n = 6), tumors were illuminated for 300 to 600 seconds through percutaneously inserted optical fibers with a light dose of 60 J/cm of diffuser (740 nm). Tumor necrosis at 1 month after PDT was achieved in all treated lesions. Laser treatment was associated with mild pain (n = 8) and transient subclinical hepatotoxicity (n = 21). In one patient, PDT damage to the pancreas was inflicted, and in another patient, PDT damage of the skin occurred, but no serious clinical complications from PDT were reported. Administration of .6 mg/kg of mTHPBC led to transient phlebitis in 10 patients, and 3 patients experienced mild skin phototoxicity after excess light exposure. Colorectal liver metastases that are ineligible for resection can be safely and effectively treated with interstitial mTHPBC-based PDT.