BACKGROUND Hepatocellular carcinoma (HCC) is the most common primary malignant liver tumor, usually arises in the setting of liver cirrhosis (LC), and has a poor prognosis. The recently discovered Th2-cytokine interleukin-33 (IL-33) is a possible mediator in pancreatic and gastric carcinogeneses. IL-33 binds to its receptor and to soluble ST2 (sST2), which thereby acts as a regulator. The role of IL-33 and sST2 in HCC has not been elucidated yet. METHODS We conducted a case-control study with 130 patients and 50 healthy controls (HCs). Sixty-five patients suffered from HCC and 65 patients had LC without HCC. We assessed serum IL-33 and sST2 levels and their association with established prognostic scores, liver function parameters, and overall survival (OS). RESULTS No significant difference in IL-33 serum levels was found in HCC compared to LC and HCs. IL-33 levels did not correlate with OS, liver function parameters, the Model for End-Stage Liver Disease (MELD) score, or the Cancer of the Liver Italian Program (CLIP) score. sST2 levels were significantly elevated in LC and HCC patients compared to HCs (P < .0001). Mean sST2 levels in LC were higher than in HCC (P < .0001), but a significant association with OS was only observed in the HCC group (P = .003). sST2 in HCC correlated with the CLIP score, the MELD score, and liver function parameters. CONCLUSIONS In the present study, the serum concentration of sST2 was associated with OS of HCC. Therefore, sST2 may be considered as a new prognostic marker in HCC and is worth further evaluation.