Results of clinical trials with bevacizumab in combination with chemotherapy have shown an improvement in survival of patients with non-small cell lung cancer and an increase in the time until their tumours progressed. As a result of these trials, bevacizumab has been registered for the treatment of advanced non small cell lung cancer in a number of countries including the United States and the European Union. The main aim of this study, is to explore if it is possible to use special ‘markers’ which are produced by the cancer cells (known as biomarkers) to predict how well patients will respond to the combination of bevacizumab and chemotherapy. These biomarkers are molecules which are released into the blood by the tumour, or may be attached to the tumour cells. It is hoped that this new information could be used to help doctors to decide which patients might get the most benefit from treatment with bevacizumab and chemotherapy in the future. Approximately 300 patients with advanced lung cancer will be randomised into this phase II trial. Patients will be randomly and equally assigned to two treatment groups: carboplatin based chemotherapy in combination with either bevacizumab 7.5mg/kg, or bevacizumab 15mg/kg. All study drugs will be administered intravenously for a maximum of 6 cycles (1 cycle = 3 weeks) of therapy or until disease progression. If patients do not experience disease progression after 6 cycles they will be prescribed Bevacizumab at the dose they were randomised to. Bevacizumab administration will continue until the patient progresses. During the study, blood samples will be taken for analysis of biomarkers or changes in certain components of blood that correlate with the disease activity, efficacy or safety of the treatment. Once patients have progressed they will be taken off study medication and followed up for survival analyses.