Anemia is a poor prognostic factor for stage I non-small cell lung cancer (NSCLC) patients treated with Stereotactic Body Radiation Therapy (SBRT).

•This study reports on the prognostic ability of haematological parameters for the largest known biopsy-proven stage-I medically inoperable cohort treated with SBRT.•After SBRT, the median values of Hb, ALC, ANC and TPC declined whereas the NLR and the PLR increased as compared to pre-SBRT.•Anemia along with other parameters was found to be a poor prognostic factor for local control despite treatment with SBRT to doses of >100 Gy BED.•Simple and minimally invasive methods like a peripheral blood sample can provide prognostic information even for stage-I NSCLC patients.•Patient, tumor and treatment factors along with molecular markers should be used to create risk stratification models that can guide therapy.