Nineteen chronic schizophrenics (8 males and 11 females) showed at least one spike and wave complex (SpW) in their rested-awake EEGs during long-term neuroleptic treatment. The age at the first appearance of the SpW ranged from 16 to 60 years, and the duration of neuroleptic medication preceding its appearance was from 1 to 35 years. Two types of SpW waveform were discriminated; one was a diffuse high voltage isolated 3.5-4 Hz SpW complex, and the other a diffuse moderate voltage 5-6 Hz SpW burst. In EEG studies repeated over the long-term, the presence of SpW was transient in 11 cases, intermittent in 5 cases, and continuous in 3 cases. Three patients had generalized tonic-clonic clinical seizures; two of their EEGs did not show SpW until after the onset of seizures. All three responded well to adjunctive anticonvulsant therapy. The other 16 patients exhibited SpW but did not have clinical seizures with or without prophylactic use of anticonvulsants. The SpW in the EEG of chronic schizophrenics might be an indicator of predisposition for seizure, but it is not a good predictor of seizure.