Several malignant tumors accumulate hyaluronan, a matrix component suggested to promote cancer cell migration and growth. To explore the potential clinical importance of this concept, we assessed the hyaluronan levels in epithelial ovarian cancer. A biotinylated affinity probe specific for hyaluronan was prepared and applied to histological sections of 309 epithelial ovarian cancers and 45 matched metastatic lesions. The staining was scored according to the percentage area of strong hyaluronan signal of total peri- and intratumoral stroma as low (<35%), moderate (35-75%), or high (>75%). Low, moderate, and high levels of stromal hyaluronan were observed in 95, 116, and 98 carcinomas, respectively. The high stromal hyaluronan level was significantly associated with poor differentiation, serous histological type, advanced stage, and large primary residual tumor, whereas it was not correlated with high CD44 expression on cancer cells. The 5-year outlook of the disease deteriorated with increasing stromal hyaluronan levels for both overall (45% versus 39% versus 26%; P = 0.002) and recurrence-free (66% versus 56% versus 40%; P = 0.008) survival. High levels of stromal hyaluronan were more frequent in metastatic lesions than in primary tumors (z = -3.9; P = 0.0001). In Cox's multivariate analyses, high level of stromal hyaluronan was an independent prognostic factor in all patients, as well as in stage-specific subgroups. These results suggest that stromal hyaluronan accumulation may be a powerful enhancer of tumor progression and, as such, provides a novel, independent prognostic marker and a potential target of therapy.