PURPOSE: To investigate the clinical application of flattening filter free (FFF) beams at maximum dose rate for stereotactic body radiotherapy (SBRT). METHODS AND MATERIALS: Patients with tumors in the lung or abdomen were subjected to SBRT using 6MV FFF or 10MV FFF beams. For each patient, three plans were calculated using 6MV flattened, 6MV FFF, and 10MV FFF beams. Treatment times were recorded and analyzed, and tumor displacements were assessed by pre- and post-treatment cone beam computed tomography (CBCT). RESULTS: Altogether, 26 patients (16 lung, 10 abdominal tumors) were treated. The average dose rate per patient ranged from 442 to 1860MU/min. Beam-on time was on average 1.6min (1SD=0.6min), with the total treatment times recorded at 18.5min (1SD=3.5min). The time advantage of using FFF beams was dose-dependent and started at 4Gy for 6MV FFF and at 10Gy for 10MV FFF beams. The average of the tumor displacements during treatment was 2.0mm (1SD=1.0mm). CONCLUSIONS: SBRT using FFF beams is time efficient and associated with excellent patient stability. According to Van Herk's formula, ITV-PTV margins of 6mm are sufficient in our patient cohort. Further studies are necessary to assess clinical outcome and toxicity.