BACKGROUND: Studies on Cox-2 and Foxp3+ regulatory T cells (Treg) in hepatocellular carcinoma (HCC) showed that Treg suppress local immune response in a Cox-2-dependent fashion. AIMS: To investigate Cox-2 expression, Foxp3+ Treg infiltration and CD4+ T cell frequency in HCC tumors. METHODS: Tumors and the corresponding nontumor hepatitis B virus-related liver tissues from 40 HCC patients with hepatitis B virus infection, plus 10 liver samples from patients with hemangioma as controls, were assessed for Cox-2 expression, Foxp3+ Treg and total CD4+ T cell numbers using immunohistochemistry. Serum TGF-beta1 was assessed by ELISA. RESULTS: Reduced Cox-2 expression, increased Treg and increased CD4+ T cells were shown in tumor as compared with nontumor tissues. Moreover, of 40 tumor tissues, 23 that expressed Cox-2 showed increased Foxp3+ Treg and reduced CD4+ T cells compared with the remaining 17 that did not express Cox-2. Correlation analyses showed that within tumors Treg infiltration correlated positively with Cox-2 expression, and that Treg infiltration or Cox-2 expression correlated negatively with CD4+ T cells. Additionally, serum TGF-beta1 was higher in HCC patients than in controls. CONCLUSION: Within tumors, Cox-2 expression, Treg infiltration and CD4+ T cell frequency were increased, and the Cox-2 expression correlated positively with Treg infiltration and negatively with CD4+ T cell frequency.