To determine the optimal screening interval for detecting small (< 20 mm) hepatocellular carcinoma (HCC) in a high-risk group using multiphase contrast-enhanced computed tomography (CECT), we evaluated the growth rate of primary single HCC. Forty-nine primary single HCC cases were reviewed. CECT screening was performed more than two times preceding to the diagnosis in 29 cases, and HCC nodule was identified at least two times in 22 cases. The initial nodule sizes ranged between 3 and 30 mm. Doubling time of tumor volume ranged from 34.8 to 496.4 days, with a geometric mean of 93.5 days, and a 95% lower threshold value of 27.1 days. It means that HCC will not double in diameter within 3 months. Therefore CECT screening at intervals of 3 months will detect new nodules at 10-20 mm in size and CECT screening at intervals of longer than 3 months will detect new nodules but they might be larger than 20 mm in size.