The expression of claudins 1, 2, 3, 4, 5 and 7 was studied in prostate adenocarcinoma and compared to that of non-neoplastic epithelium and Gleason score of the tumors. Additionally, RT-PCR was performed for claudins 2 and 5 in three cases. Strong immunoreactivity of claudins 1, 3, 4 and 7 was seen in prostate adenocarcinoma while expression of claudins 2 and 5 was weaker. In relation to non-neoplastic glands, expression of claudins 2 and 5 was diminished. There was a significant association between the Gleason score and claudin 1 and 5 expression, these claudins were more strongly expressed in tumours with a lower Gleason score. A combined lowered claudin expression was associated with a high Gleason score and a poor prognosis. According to the results, there is a strong claudin expression in prostate adenocarcinoma, especially for claudins 3 and 4. In contrast, claudin 2 was low in neoplastic cells compared to non-neoplastic epithelium, suggesting downregulation of synthesis or altered metabolism/assembly of this protein during carcinoma development. The association of lowered claudin 1 and 5 expression and lowered overall claudin expression with tumours of a higher Gleason score suggest that claudins may modulate the histologic features of these tumors and in this way influence their biological behaviour.