Secondarily generalized seizures are usually partial seizures evolving into generalized seizures, most often with tonic-clonic convulsions. The partial seizures which were once limited to one hemisphere of the brain progress to encompass the entire brain bilaterally, causing a generalized seizure. The clinical nature of a secondarily generalized seizure usually does not differ from that of the initial, originating seizure.

Secondarily generalized seizures are predominant in 16% of all children and 9% of all adults with seizure disorders. Most people with complex partial seizures and many with simple partial seizures will experience a secondarily generalized seizure at some point. When they occur frequently, the chances for future partial seizures may be increased. Secondarily generalized seizures occur infrequently and are easily controlled by anti-epileptic medication.