BACKGROUND To assess for variables predicting pulmonary function test (PFT) changes after stereotactic body radiotherapy (SBRT) for medically inoperable stage I lung cancer. METHODS We reviewed 92 consecutive patients undergoing SBRT for stage I lung cancer between February 2004 and August 2007. A total of 102 lesions were treated using prescriptions of 20 Gy x 3 (n = 40), 10 Gy x 5 (n = 56), and 5 Gy x 10 (n = 6). Institutional practice was 10 Gy x 5 before March 1, 2006 before changing to 20 Gy x 3 to conform to RTOG 0236 unless otherwise dictated clinically. RESULTS Median pretreatment forced expiratory volume at 1 second (FEV1) was 1.21 liter (50% of predicted) and median diffusion capacity to carbon monoxide (DLCO) was 56.5. There was no significant overall change in PFT's after SBRT. Individual patients experienced both substantial improvements and declines (10% declined at least 14% predicted FEV1% and 19% predicted DLCO). The mean change in FEV1 was -0.05 liter (range, -0.98 to +1.29 liter; p = 0.22) representing -1.88% predicted baseline FEV1 (range, -33 to + 43%; p = 0.62). DLCO declined 2.59% of predicted (range, -37 to +33%; p = 0.27). Conformality index, V5 and V10 were associated with individual patient changes in FEV1% (p = 0.033, p = 0.0036, p = 0.025, respectively), however, correlations were small and overall treatment dose did not predict for changes (p = 0.95). There was no significant difference in FEV1 (p = 0.55) or FEV1% (p = 0.37) changes for central versus peripheral locations. No factors predicted for individual changes in DLCO. Patients with FEV1% below the median of the study population had significantly longer overall survival (p = 0.0065). Although patients dying of cardiac disease died earlier than those dying of other causes, FEV1% below median was not associated with a lower risk of dying of cardiac disease or with lower Charlson comorbidity index. CONCLUSIONS (1) SBRT was well tolerated and PFT changes were minimal. (2) Central lesions were safely treated with 50 Gy.