BACKGROUND: Corticosteroids (CS) are a common treatment prescribed for dogs with brain tumors, but little data characterizing the clinical and neuroradiologic effects of CS treatment exist. HYPOTHESES: Decreases in peritumoral brain edema (PBE) would be observed in dogs with brain tumors treated with CS and anticonvulsants, and decreases in edema would be accompanied by clinical improvement. ANIMALS: Fifty dogs with gliomas and 22 with meningiomas. MATERIALS AND METHODS: Retrospective case series. Dogs with brain tumors receiving treatments with CS and anticonvulsants underwent pre- and posttreatment clinical and brain magnetic resonance imaging (MRI) examinations within a 3-week follow-up period, and peritumoral edema and tumor volumes were calculated from each MRI study. Dogs were characterized as clinical responders or nonresponders independent of imaging results based on longitudinal changes in clinical findings and owner-reported quality of life (QOL). Clinicopathologic and MRI variables were compared pre- and posttreatment by clinical response. RESULTS: At follow-up, 23/50 (46%) of dogs with glioma and 15/22 (68%) with meningioma were classified as clinical responders, and 23/50 (46%) of gliomas and 14/22 (64%) of meningiomas had decreases in edema volume. Responders had significantly larger decreases in edema and mass effect than nonresponders. Decreases in tumor volumes occurred in approximately 25% of gliomas. CONCLUSIONS AND CLINICAL IMPORTANCE: Peritumoral brain edema was decreased in 50%-60% of gliomas and meningiomas in dogs at follow-up, and amelioration of edema often was accompanied by improved neurological signs and QOL. Corticosteroids may also clinically benefit dogs without PBE and can influence MRI surrogates used to determine glioma therapeutic responses.