Non-small cell lung cancer (NSCLC) is the commonest cause of cancer mortality worldwide. Growth factor receptor signalling pathways constitute an important mediator for tumor growth and proliferation. PTEN and pAKT play important roles in regulating signal transduction along this pathway. Separate cohorts of stage I (n=25) and stage IV (n=34) NSCLC were examined by immunohistochemistry for PTEN and pAKT expression. There was no correlation between PTEN expression and pAKT expression and neither were associated with age, sex or smoking status. Patients with stage IV disease who overexpressed pAKT (at least 2+) or were PTEN-null had poorer overall survival and progression-free survival. This suggests that PTEN-null or pAKT-positive tumors constitute more aggressive tumors whose clinical course is not altered by therapy. There was no difference in the clinical outcome for stage I disease by PTEN or pAKT expression. A greater proportion of the stage IV patients had PTEN-null disease compared to the stage I cohort, suggesting that loss of PTEN is important in the tumor biology of advanced disease. Loss of PTEN or overexpression of pAKT predicts for an aggressive subset of lung tumors that have a poor prognosis. This will allow identification of a poor prognosis subset that can be targeted with novel treatments that either restore PTEN function or target activated AKT, mTOR and other downstream signal transduction molecules.