To clarify the role of p33ING1b in tumorigenesis and progression of head and neck squamous cell carcinoma (HNSCC), we examined the expression and subcellular localization of p33ING1b in 214 HNSCC cases in parallel with 60 dysplasia samples and 48 normal epithelium samples by immunohistochemistry, and analyzed correlations of expression of p33ING1b in HNSCC cases with clinicopathological variables, apototic index and expression of 14-3-3η, p300, p21 and PCNA. Although 12% of HNSCC cases lost expression of p33ING1b, most cases of HNSCC retained expression of p33ING1b with levels similar to those in non-cancerous epithelia. Nuclear expression of p33ING1b was significantly decreased in HNSCC compared to normal epithelia. In contrast, cytoplasmic expression of p33ING1b was found to be significantly higher in HNSCC. An abundance of p33ING1b in cytoplasm positively correlated with poor differentiation and tumor progression. Corresponding to those clinicopathogical features, high expression of p33ING1b in the cytoplasm correlated with PCNA labelling index but in contrast, that in the nuclei correlated with apoptosis. In nuclei, p33ING1b is coexpressed with p300 and p21, implying its roles in tumor suppression. Elevated expression of 14-3-3η was associated with cytoplasmic expression of p33ING1b and immunofluorescence study suggested association of p33ING1b and 14-3-3η. Among three cell lines derived from oral SCC, poorly-differentiated SAS cells showed a relatively high expression of p33ING1b in cytoplasm with increased level of 14-3-3η. The results obtained here suggest that relocation of p33ING1b from the nucleus to the cytoplasm, where the protein is tethered by 14-3-3η, participates in tumorigenesis and progression in HNSCC.