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Aquaporin-1 (AQP1) is the main water channel responsible for water transport through many epithelia and endothelia. The latest evidence pointed toward an important role of this protein also in gas permeation, angiogenesis, cell proliferation and migration. In the present work we studied the expression of AQP1 by immunohistochemical staining of 92 lung biopsies from patients diagnosed with a pleuro-pulmonary tumor (71 lung and 21 pleural neoplasms). AQP1 expression was analyzed comparing the results among the different histological patterns and against 9 control cases (5 parenchyma and 4 healthy pleura). Clear staining of AQP1 was detected in 39 of the 92 tumors analyzed. In parenchyma, AQP1 was more frequently detected in primary lung adenocarcinomas (55%, P<0.001); in contrast, small cell carcinomas were the least AQP1 expressive tumors studied (93% of negative staining, P<0.05). Carcinomas analyzed in pleura (mesotheliomas and metastatic adenocarcinomas) also revealed strong expression of AQP1. High expression of this protein was detected in small capillaries in areas near or surrounding the tumor, and novel intense AQP1 immunostaining was detected over thicker alveolar walls in alveoli inside or next to the tumoral tissue regardless of the tumor type. An important role of AQP1 in tumor angiogenesis is sustained by the abundant expression of this protein in the endothelia of tumor capillaries. Further studies are necessary to elucidate the potential pathophysiological role of this protein in pleuro-pulmonary neoplasms.