The purpose of this study was to evaluate the long-term (12 months) efficacy and tolerability of imepitoin as first-choice treatment in 56 dogs suffering from idiopathic epilepsy and identify possible factors affecting the outcome. Primary treatment success (PTS) was defined as the achievement of a seizure-free interval three times longer than the pretreatment interictal interval (at least three months). Secondary treatment success (STS) was achieved by a decrease in seizure frequency ≥50 per cent compared with the pretreatment frequency. In the long-term follow-up, PTS was recorded in 14 (25 per cent) dogs and responder-dogs (PTS+STS) were 30 (54 per cent) showing significant reduction in the monthly average number of seizures (P<0.001). Median seizure frequency per month was 1.69 pretreatment and 0.3 at 12-month follow-up. Dogs with cluster seizures were significantly reduced (P=0.02). PTS at three and six months was associated with PTS (P=0.006 and <0.001, respectively) and with the status of responder dogs (P=0.002) at 12-month follow-up. Dogs aged >36 months at the start of imepitoin treatment had a positive association to become responder dogs (P<0.001) and achieve PTS (P=0.004). 16 dogs (29 per cent) discontinued imepitoin due to its inefficacy. The receiver operator curve highlighted ≥19 mg/kg twice a day as the most effective minimal dosage. Mild and transient side effects were observed in 16 dogs (29 per cent).