We examined serial changes of tumor images in a patient with hepatocellular carcinoma. The hepatocellular carcinoma was initially detected as a homogeneous low-echo area with unclear margins, which was not enhanced by contrast media on computed tomograms, and did not reveal any vascular abnormalities on hepatic angiography. About 11 months later, the tumor growth accelerated, with a parallel increase in serum alpha-fetoprotein levels, and the ultrasonographic features of the tumor changed from a homogeneous low-echo area to a mixed low- and high-echo area with a peripheral low-echo zone. Hepatic angiography revealed a hypervascular tumor at this time. The present case indicates that tumor growth and imaging patterns of hepatocellular carcinoma are closely related to vascularization of the tumor.