BackgroundData on acute toxicities after stereotactic radiotherapy (SRT) for brain metastases, including multiple and large lesions, are lacking. We aimed to evaluate the incidence and nature of toxicities immediately after SRT using a linear accelerator.MethodsThis retrospective study reviewed the medical records of 315 patients with brain metastases treated with SRT at our institution between May 2019 and February 2022. In total, 439 SRT sessions were performed for 2161 brain metastases. The outcome of interest was immediate side effects (ISEs), defined as new or worsening symptoms occurring during SRT or within 14 days after the end of SRT.ResultsGrade ≥ 2 and ≥ 3 ISEs occurred in 16 (3.6%) and 7 (1.6%) cases, respectively. Among 63 treatments for 10 or more lesions (range: 10–40), 1 (1.6%) ISE occurred. Among 22 treatments for lesions with a maximum tumor volume of > 10 cc, 2 (9.1%) ISEs occurred. Grade ≥ 3 ISEs included 1, 4, 1, and 1 cases of grade 3 nausea, grade 3 new-onset partial and generalized seizures, grade 3 obstructive hydrocephalus, and grade 5 intracranial hemorrhage, respectively. ISEs were more common in patients with a larger maximum tumor volume, primary sites other than lung and breast cancer, and pre-treatment neurological symptoms.ConclusionSRT using a linear accelerator for brain metastases, including multiple and large lesions, is safe, with a low incidence of ISEs. Serious complications immediately after SRT are rare but possible; therefore, careful follow-up is necessary after treatment initiation.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13014-023-02262-z.