Background Prediction of the tumor growth rates is clinically important in patients with hepatocellular carcinoma (HCC), but previous studies have presented conflicting results and generally lacked radiologic evaluations. Purpose To evaluate the percentage of rapidly growing early-stage HCCs in each Liver Imaging Reporting and Data System (LI-RADS) category and to identify prognostic factors associated with rapid growth. Materials and Methods Retrospective study of patients with risk factors for HCC and those with surgically proven early-stage HCC who underwent two or more preoperative multiphasic CT or MRI examinations between January 2016 and December 2020. LI-RADS categories were assigned according to the baseline CT or MRI results. The tumor volume doubling time (TVDT) was calculated from the tumor volumes measured at the two examinations. The growth rate was classified as rapid (TVDT < 3 months), intermediate (TVDT = 3-9 months), or indolent (TVDT > 9 months). The percentage of rapidly growing HCCs was compared among the LI-RADS categories, and multivariable logistic regression was used to identify factors associated with rapidly growing HCC. Results In 322 patients (mean age, 61 years ± 9 [SD]; 249 men) with 345 HCCs (30 LR-3, 64 LR-4, 221 LR-5, and 30 LR-M category), the median TVDT of HCC was 131 days (IQR, 87-233) and 27.0% of HCCs showed rapid growth. The growth rates differed among the LI-RADS categories, with a higher percentage of rapidly growing HCCs observed for LR-M HCCs than for LR-3 (70.0% vs 3.3%, P < .001), LR-4 (70.0% vs 12.5%, P < .001), or LR-5 (70.0% vs 28.5%, P < .001) HCCs. An α-fetoprotein level greater than 400 ng/mL (adjusted odds ratio [OR], 2.54; 95% CI: 1.16, 5.54; P = .02), baseline tumor diameter (adjusted OR, 0.65; 95% CI: 0.48, 0.87; P = .004), and LR-M category (adjusted OR, 9.26; 95% CI: 3.70, 23.16; P < .001) were independently associated with higher odds of rapid growth. Conclusion Among early-stage HCCs, LR-M category was an independent factor for rapid growth, observed in 70% of HCCs. © RSNA, 2024 Supplemental material is available for this article.