We describe a case of histologically proven hepatobiliary cystadenocarcinoma with an emphasis on the longterm clinical course. The patient was a 75-year-old man who had been diagnosed as having a simple hepatic cyst at our hospital and had been followed up by abdominal sonography (US) for about 10 years. However, the lesion subsequently showed a sudden increase in size and a marked change in US findings, i.e., from a benign hepatic cyst to a complete solid lesion. Contrast-enhanced US revealed the lesion to be filled with fine vessels. This rapid change led us to strongly suspect a hepatobiliary cystadenocarcinoma and, thus, left lateral segmentectomy was performed. The patient is doing well 6 months after the surgical treatment.