Extensive studies have demonstrated that the Akt/AKT1 pathway is essential for cell survival and inhibition of apoptosis; however, alterations of Akt/AKT1 in human primary tumors have not been well documented. In this report, significantly increased AKT1 kinase activity was detected in primary carcinomas of prostate (16 of 30), breast (19 of 50), and ovary (11 of 28). The results were confirmed by Western blot and immunohistochemical staining analyses with phospho-Ser473 Akt antibody. The majority of AKT1-activated tumors are high grade and stage III/lV (13 of 16 prostate, 15 of 19 breast, and 8 of 11 ovarian carcinomas). Previous studies showed that wild-type AKT1 was unable to transform NIH3T3 cells. To demonstrate the biological significance of AKT1 activation in human cancer, constitutively activated AKT1 (Myr-Akt) was introduced into NIH3T3 cells. Overexpression of Myr-Akt in the stably transfected cells resulted in malignant phenotype, as determined by growth in soft agar and tumor formation in nude mice. These data indicate that AKT1 kinase, which is frequently activated in human cancer, is a determinant in oncogenesis and a potential target for cancer intervention.