Variability in gross tumour volume (GTV) definition is a major source of systematic error in conformal radiotherapy. This prospective study assesses the role of multidisciplinary collaboration between oncologists and radiologists in defining lung cancer volumes. Twenty patients with non-small cell lung cancer due to receive three-dimensional conformal radiotherapy formed the study population. GTVs were defined by a radiologist (GTVrad) and an oncologist (GTVonc) using available clinical information and imaging. A collaborative meeting was then held to agree on a final, common GTV (GTVfin) to be used for treatment planning, and differences analysed. The collaboration changed the GTV in 19/20 patients with a total of 50 regions being edited. Changes made were categorized as (a) differentiation of tumour from atelectasis or ground glass shadowing, (b) separation of tumour from vasculature, and (c) defining mediastinal extent of tumour. Oncologists were more confident in the GTVfin than the GTVonc. The radiologist took longer to define the GTV than the oncologist. Real-time collaborative GTV definition by a radiologist and oncologist is practical and feasible. This approach allows specific areas of uncertainty to be categorized and focussed on, reducing systematic error in GTV definition. The physician's approach to risk and decision making for each patient may also play a role.