Interleukin-4-induced gene 1 (IL4I1) is a secreted L-phenylalanine oxidase that deaminates phenylalanine to phenylpyruvate, releasing H2O2 and NH3 in the process. IL4I1 is mainly secreted by inflammatory antigen presenting cells and is involved in the regulation of adaptive immune responses. Furthermore, it has been re-ported that IL4I1 is overexpressed in a variety of tumor tissues and affects tumor development. We explored the expression patterns, correlation with clinical traits and prognostic value of IL4I1 using public databases and microarray data from sample banks. Subsequently, we used the downloaded data to score tumor stromal cells and immune cell infiltration and analyzed the correlation between IL4I1 and immune cells or immune-related molecules in combination with TIMER2.0 and GEPIA databases. The analysis showed that IL4I1 was associated with the infiltration status of various immune cells. Finally, stable IL4I1-overexpressing hepatocellular carcinoma (HCC) cell lines were established to investigate the effect of IL4I1 on cell proliferation and motor capacity. All of these results suggest that IL4I1 is a potential biomarker and therapeutic target.