Although there is no definitive evidence that hepatocellular carcinoma (HCC) screening in high-risk groups improves survival, many physicians screen high-risk populations with various tools such as alpha-fetoprotein (AFP) and ultrasonography (USG). The aim of this study was to clarify clinical differences between HCC patients diagnosed by surveillance and those with incidentally detected HCC. Two hundred and seventy-one Japanese patients with HCC diagnosed between January 1991 and December 2001 were recruited. They were categorized into two groups: 178 patients (group 1) had subclinical HCC diagnosed by surveillance and 93 patients (group 2) presented with incidentally detected HCC. The tumor size was significantly smaller in group 1 compared to that of group 2 (2.8 vs. 5.6 cm; P<0.0001). A significantly higher proportion of patients in group 2 had multiple HCC and portal vein infiltration when compared to group 1. Eighty-six (48.3%) group 1 patients and 16 (17.2%) group 2 patients underwent local ablation treatment, which is a curative treatment available for small HCCs (P<0.0001). The cumulative actuarial survival rate was significantly higher in group 1 than in group 2 (P=0.0091). Early detection of HCC by surveillance may contribute to a greater chance of receiving effective treatment and prolonged survival, although a further prospective, randomized study is needed.