We performed serial hemodynamics in 15 patients with 21 well-differentiated hepatocellular carcinomas. The total length of the observation period ranged from 129 to 678 days (median, 368). We investigated both arterial and portal blood flow at intervals of at least 4 months. Arterial blood flow was measured with carbon dioxide-enhanced ultrasonography (US), and portal blood flow was measured with computed tomographic arterial portography (CTAP). None of the tumors were hypervascular on the initial study; however, by the final study nine nodules (42.9%) had become homogeneously hypervascular, two (9.5%) had become partially hypervascular, and the others did not change. Of 10 nodules without portal blood supply on any study, 8 became hypervascular by the final study. Of 11 nodules with portal blood supply on the initial study, a follow-up study showed no change in portal flow in 6 and the loss of portal flow in 5. Four of these five became hypervascular, with a documented loss of portal flow before the increase in arterial flow. The doubling time of tumors with a homogeneous increase in arterial blood flow ranged from 89 to 333 days (median, 172), whereas the doubling time of other tumors ranged from 227 to 607 days (median, 392). Thus, growth rate and vascularity in well-differentiated hepatocellular carcinoma are closely correlated. When the well-differentiated hepatocellular carcinoma has portal blood flow and is not hypervascular, it grows slowly. On the other hand, when it loses portal blood flow and becomes hypervascular, it grows rapidly.