Previously reported cases of primary hepatic small-cell carcinoma were all detected at progressed state with associated symptoms. Therefore, the natural course of primary hepatic small-cell carcinoma remains unknown. This case shows the natural course of primary hepatic small-cell carcinoma. We detected a 1.2 cm hypodense nodule 6 months ago in a patient with cirrhosis who had been taking entecavir. It was suspected to be a regenerating or degenerating nodule. Three months later, liver computed tomography (CT) revealed that the mass was increased to 2.1 cm with the same characteristics. The patient wanted to do a follow-up CT scan after 3 months instead of a biopsy. Another 3 months later, the mass was markedly increased, involving the whole left lobe and was confirmed as small-cell carcinoma on biopsy. Here, we report the first case of primary hepatic small-cell carcinoma developed during treatment for chronic hepatitis B with cirrhosis.