Tumor implantation along the needle tract following percutaneous procedures under ultrasonographic guidance for hepatocellular carcinoma (HCC) and pancreatic carcinoma (PC) has been well documented. The purpose of the present study was to investigate the correlation between the procedure, the pathologic differentiation of the primary tumor, and the treatment after implantation. Between July 1992 and March 2000, HCC patients (n=372) who underwent biopsy, percutaneous ethanol injection (PEI) therapy and percutaneous microwave coagulation therapy (PMCT) and PC (n=73) patients who underwent biopsy were retrospectively studied. Needle tract implantation was found in six of the HCC patients (1.6%) and one of the PC patients (1.4%). The interval to diagnosis ranged from 5 to 25 months (mean +/- SD 11.2 +/- 7.6 months) in the HCC patients. The needle tract implantation was evident for all procedure types in these patients (two after PEI alone, two after both biopsy and PEI, and one after PMCT) and for each degree of pathologic differentiation of the primary tumors (well differentiated in one, moderately differentiated in two, and poorly differentiated in one). Each implanted tumor was surgically resected, with no recurrence at the focal lesion. These results suggest that needle tract implantation develops regardless of the procedure or the pathologic differentiation of the primary tumor, and that surgical resection might be effective for controlling these implanted lesions.