RATIONALE AND OBJECTIVES.Interstitial laser photocoagulation (ILP) destroys tumors thermally, using laser energy delivered from implanted optical fibers. The objectives of the study are to identify a fiber tip/delivered energy combination which produces lesions of useful size, visible on ultrasound (US) during ILP, and to compare ILP lesions and their US images. METHODS.Hepatic ILP was performed at laparotomy in six pigs, using three different fiber tips (cylindrical diffusing, spherical diffusing, plane-cut). US images were obtained during ILP, immediately after (“early” images), and before the animals were killed (2-2.5 hours, “late” images). Actual lesions were assessed histopathologically. RESULTS.Few US changes were seen around cylindrical diffusing and spherical diffusing tips until tip destruction. Plane- cut tips, at 1.5 to 2.0 W, produced prominent US images of the 1- to 2-cm thermal lesions. Early images tended to overestimate necrosis. Late images approximated necrosis. CONCLUSION.For US-controlled ILP, plane-cut tips are better than currently available cylindrical diffusing or spherical diffusing tips. Lesion image growth periods might enable control of lesion size. Further studies are needed to determine the consistency of the described relationship between lesion images and actual lesions.