Interstitial laser photocoagulation (ILP) is becoming an established treatment for hepatic metastases. Per-procedural ultrasound does not accurately define the tissue coagulated and dynamic computed tomography, while an accurate form of assessment, must be delayed until 24 hours after therapy; it too is, therefore, no use for per-procedural confirmation of total tumor ablation. By virtue of its good contrast, spatial and temporal resolution and the fact that T1 weighted images are temperature sensitive magnetic resonance imaging (MRI) is, theoretically, applicable to the per-procedural guidance of ILP. Our ex vivo porcine, and in vivo rat liver experiments showed good imaging-pathological agreement. We have treated with ILP two patients suffering from hepatic metastases using MRI guidance. Four to eight 400 micrometers optical fibers were passed through the lesion down Teflon sheaths under local anaesthetic and intravenous sedation using ultrasound guidance. Sets of 400 second laser burns at 1.5 - 2.0 W per fiber were then performed, with a 1 cm fiber withdrawal between each set, until the entire tumor had been coagulated. Therapy was monitored with dynamic MRI using a T1 weighted FLASH sequence. MRI showed an enlarging bright rimmed low signal area which correlated with the region of devascularization seen on enhanced helical computed tomography at 24 hours. MRI can monitor ILP of hepatic metastases and confirm tumor ablation at the time of treatment.