The phosphatidylinositol 3-kinase (PI3K)-AKT and RAS-MAPK pathways are deregulated in a wide range of human cancers by gain or loss of function in several of their components. Our purpose has been to identify genetic alterations in members of these pathways in prostate cancer. A total of 102 prostate tumors, 79 from prostate cancer alone (group G1) and 23 from bladder and prostate cancer patients (G2), are the subject of this study. In 20 of these 23, the bladder tumors were also analyzed. PIK3CA, KRAS, BRAF and AKT1 mutations were analyzed by direct sequencing, and BRAF also by pyrosequencing. PIK3CA quantitative mRNA expression and fluorescence in situ hybridization (FISH) gains were tested in 25 and 32 prostate tumors from both groups (G1 and G2), respectively. Immunohistochemistry for pAKT was performed in 55 prostate tumors. Of 25 prostate tumors, 10 (40%) had PIK3CA mRNA overexpression that was statistically associated with Gleason score ≥ 7 (P=0.018). PIK3CA copy gain was detected in 9 of 32 (28%) prostate tumors. Of 20 bladder tumors, 3 (15%) displayed mutations in PIK3CA, KRAS and AKT1, the corresponding prostate tumors being wt. We also detected a previously not reported PIK3CA polymorphism (IVS9+91) in two prostate tumors. In all, 56% of prostate tumors overexpressed pAKT. There is a statistical association (P<0.0001) of strong pAKT immunostaining with high Gleason score, and with PIK3CA alterations (mRNA overexpression and/or FISH gains). PIK3CA gene is deregulated by mRNA overexpression and DNA gain in ∼ 40 and 28% of prostate tumors, respectively. High-grade prostate tumors are associated with PIK3CA mRNA overexpression, but not with FISH status. PIK3CA, BRAF, KRAS and AKT1 mutations are very infrequent events in prostate tumors. However, PI3K signaling pathway is activated by PIK3CA FISH gain and/or mRNA overexpression, leading to an increased pAKT protein expression.