Imaging modalities such as CT and ultrasound are highly accurate for diagnosing simple hepatic cysts, however, the distinction between cystadenoma and a simple hepatic cyst complicated by intracystic hemorrhage has been reported to be difficult [
3]. The measurement of serum and cyst fluid CA19-9 levels has been reported to be helpful in distinguishing between a hemorrhagic simple cyst and cystadenoma or cystadenocarcinoma [
4,
5]. However, serum and cyst fluid CA19-9 levels were increased in our patient with a simple hepatic cyst before treatment, challenging this suggestion. Elevated serum and cyst fluid CA19-9 levels in our patient before treatment may be due to an infected hepatic cyst. Yoshida
et al. also observed elevated serum and cyst fluid CA19-9 levels in a patient with an infected hepatic cyst [
2], and in a patient with a simple hepatic cyst complicated with intracystic hemorrhage [
6]. Further, elevated serum CA19-9 levels were found in a patient with an inflammatory pseudotumor of the liver [
7]. Sawabu
et al. reported that serum CA19-9 levels in patients with cholelithiasis complicated by cholangitis frequently showed markedly high values, and serum CA19-9 levels were rapidly decreased and normalized by amelioration of inflammation [
8], indicating a significant association between inflammation of the hepatobiliary system and CA19-9 levels. In our patient, cyst fluid CA19-9 levels were elevated concomitant with increased flow of necrotic cyst wall cells after the minocycline instillation, suggesting that CA19-9 may originate from the hepatic cyst wall cells. Inflammation, including infection and the minocycline instillation-induced tissue injury may induce necrosis of the hepatic cyst wall cells, and may consequently increase serum and cyst fluid CA19-9 levels. This study revealed that the treatment of a simple hepatic cyst by instillation of minocycline hydrochloride significantly decreased the serum CA19-9 levels, suggesting that serum CA19-9 may also originate from the hepatic cyst.