PURPOSE: To determine the imaging characteristics of well-differentiated hepatocellular carcinomas (HCCs), as seen at triple-phase helical CT. MATERIALS AND METHODS: Thirty-eight well-differentiated HCCs in 29 patients who underwent surgical resection comprised our study population. Triple-phase helical CT images were obtained at 30 seconds, 70 seconds, and 3 minutes after contrast injection, and HCCs were divided into four groups according to their size: smaller than 1 cm (n=12), 1-2 cm (n=15), 2-4 cm (n=8), and larger than 4 cm (n=3), The enhancement patterns of HCCs at triple-phase helical CT were retrospectively analysed. RESULTS: In the smaller than l cm group, the most common enhancement pattern at all three phases was iso-attenuation (n=8; 67%). In the 1-2 cm group, iso-attenuation (n=10; 67%) was most common during the arterial phase and slightly low attenuation during the portal and equilibrium phases while in the 2-4 cm group, high attenuation (n=6; 75%) was most common during the arterial and portal phases, and low attenuation during the equilibrium phase (n=6; 75%). Two (67%) of three cases in the larger than 4 cm group demonstrated low, including fatty, attenuation. The detection rate was 33% (4/12) in the smaller than 1 cm group, 87% (13/15) in the 1-2 cm group and 100% (11/11) in the 2-4 cm and larger than 4 cm groups. CONCLUSION: At triple-phase helical CT, the enhancement patterns of well-differentiated HCCs were diverse, and the detection rate of those larger than 1 cm was 92% (24/26).