On a retrospective basis, the response to adding chronic oral bromide (BR) to phenobarbital (PB) administration in 23 refractory canine idiopathic epileptics between 1986 and 1991 was studied. The mean age for an observed first seizure was 24 months (range 7 to 72) for all dogs. Thirteen (57%) dogs were males with no breed predisposition observed. All dogs were diagnosed as having idiopathic epilepsy based on normal metabolic and neurologic diagnostic evaluations. Dogs were evaluated before BR therapy for a mean time of 22 months (range 5 to 75 months). Seventeen dogs (74%) received multiple antiepileptic drugs (AEDs) before BR therapy. All animals were maintained on PB at least 4 months before the onset of BR therapy, with a mean trough serum concentration of 37.8 mcg/mL and no improvement in seizure severity or recurrence. Twelve dogs presented with generalized isolated seizures and 11 with generalized cluster seizures (two or more seizures within 24 hours) as their first seizure. The effects of BR therapy were evaluated for a mean time of 15 months (range 4 to 33), with 17 dogs (74%) followed for 12 or more months. The mean BR serum concentration for the 0 to 4 months time period was 117 mg/dL compared with 161 mg/dL for the greater than 4 months period. Overall, response to BR therapy was associated with a reduction in the total number of seizures in 83% of the dogs when compared with their respective pre-BR period. For those followed for 1 year after BR, there was a 53% reduction in the number of seizures compared with the previous 12 months. Furthermore, owners reported a decrease in seizure intensity (65% of dogs) and change to a less severe seizure type (22% of dogs) in those dogs that continued to have seizures. Seizure-free status was obtained in 26% of the dogs with protection continuing up to 31 months in one dog. No correlations could be determined between response to BR and either age of onset of the first seizure or interval from the first AED therapy to BR therapy. Adverse effects of concomitant BR and PB therapy were polydipsia (56% of dogs), polyphagia (30% of dogs), excessive sedation (30% of dogs), and generalized ataxia (17% of dogs). As a result of BR treatment, the PB dosage was reduced in eight dogs (35%). In conclusion, concomitant BR and PB was well tolerated in dogs of this study and was effective in treating refractory canine idiopathic epilepsy, regardless of prior interval of seizure activity or previous treatment.