Reverting to the more strict definition of secondary bilateral synchrony (SBS) of Tukel and Jasper (1952), we reexamined clinical and EEG correlates of this phenomenon. SBS occurred in 57 of 10,410 consecutive patients (0.5%) recorded in our laboratory. SBS originated in the frontal lobe in 51% of patients, significantly more often than the incidence of frontal spikes among controls. The best indicator of SBS origin was the most active spike focus which localized the onset in 52 of the 57 patients (91%) and falsely identified the origin in only one. Other focal or lateralizing clinical and EEG features appeared each in a minority of patients and lateralized and/or localized SBS origin with the following incidences when present: seizure manifestations (86%), neurological examination (95%), focal delta (100%), focal or unilateral theta (100%), and offset of bisynchronous paroxysms (96%). However, the most active spike focus triggered SBS in all but two instances when these other features appeared. Reliability of such indicators of SBS origin may obviate the need for depth electroencephalography in such patients. The presence of more than one spike focus in 96% of patients and three or more foci in 77% suggests that SBS results from a complex interaction of multiple potentially epileptogenic regions, instead of spread from a single focus as previously thought.