There is paucity of data in the literature regarding the safety of combining granulocyte colony stimulating factor (G-CSF) during chemo-radiotherapy (CTRT) in lung cancer patients. The ASCO 2006 recommendations advise against use of CSFs during concomitant mediastinal CTRT. The only randomised study evaluating CSFs in this context showed significant increase in grade 3/4 thrombocytopenia and an excess of pulmonary toxic deaths. In the context of a phase II trial, 38 patients with limited-stage small cell lung cancer were randomised to receive once-daily (66 Gy in 33 fractions) or twice-daily (45 Gy in 30 fractions) radiotherapy. Radiotherapy (RT) was given concurrently with cisplatin and etoposide. G-CSF was given as primary or secondary prophylaxis or as a therapeutic measure during an episode of febrile neutropenia according to local protocols. Common terminology criteria for adverse events (CTCAE) v3.0 was used to record toxicity. Thirteen (34%) of 38 patients received G-CSF concurrently with RT. With a median follow-up of 16.9 months, there were no treatment related deaths reported. Seven (54%) patients experienced grade 3/4 thrombocytopenia and 5 (38%) experienced grade 3/4 anaemia. Thirty-one percent required platelet transfusions. No episodes of bleeding were observed. There were no cases of grade 3/4 acute pneumonitis. These data suggests that with modern three-dimensional (3D) conformal RT, G-CSF administration concurrently with CTRT does not result in the increase risk of pulmonary toxicity, but does increase the risk of thrombocytopenia. Whether the risks of thrombocytopenia are outweighed by the outcome of timely early concurrent CTRT is being evaluated prospectively in the ongoing phase III CONVERT trial (NCT00433563) in which G-CSF is permitted during thoracic irradiation.