Actinin-4 is an isoform of non-muscular alpha-actinin and actin-bundling protein. By enhancing cell motility, actinin-4 shows different biological properties from another isoform of non-muscular actinin 'actinin-1' and variable clinicopathological implications of actinin-4 have been demonstrated in some human malignancies such as breast cancers, lung cancers, and colorectal cancers. We herein described the clinicopathological and prognostic significance of actinin-4 expression in ovarian cancers. Actinin-4 expression was analyzed immunohistochemically in 265 primary ovarian carcinomas: 116 serous, 71 clear cell, 43 endometrioid, and 35 mucinous adenocarcinomas. With reference to endothelial immunoreactivity, cytoplasmic expression of actinin-4 was classified as either low (including negative) or high. Then, various parameters such as patients' characteristics, histopathological findings including E-cadherin and beta-catenin immunoreactivity, and clinical outcome, were compared between groups showing differences in the intensity or intracellular distribution of actinin-4 immunoreactivity. High expression of actinin-4 was demonstrated in 137 (57%) cases and was associated with serous histology (P=0.0075), high histological grade (P<0.0001), an advanced disease stage (P=0.036), a high degree of residual disease after initial surgery (P=0.0047), poor patient outcome (5-year survival: 52.4% in the high-expression group vs 71.9% in the low expression group, P=0.0043 by log-rank test), and also with reduced E-cadherin and preserved beta-catenin expressions (P=0.0097 and 0.017, respectively). Nuclear immunoreactivity for actinin-4 was detected in 20 (7.5%) cases and was associated with low histological grade (P=0.0079) but not with other variables. Multivariate analysis showed that high actinin-4 expression was an independent prognostic factor for overall survival, as well as a high degree of residual disease and clear-cell histology. Accumulation of actinin-4 in the cytoplasm may be related to a higher propensity for tumor invasiveness and metastasis, probably by enhancing cell motility, and could be a novel prognostic indicator for patients with ovarian carcinomas.