We studied the value of long video-split electroencephalographic monitoring (VSEEG) in detecting myoclonus in nearly SSPE and evaluated the natural history and outcome-affecting factors. The 32 newly diagnosed patients had VSEEG to detect myoclonus and its correlations with EEG periodic complexes. Disease progression was monitored by a special "outcome score'; the chi-square test and multi-variable statistics analysed the outcome score in relation to different variables, such as age at onset, sex, duration of symptoms at presentation, CSF measles antibody titre, type and interval between periodic complexes (EEG discharges). Myoclonus or atonia occurred in all patients and was time-related to the EEG periodic complexes; in 32% of patients, myoclonus or atonia were not clinically evident. The EEG periodic complexes were of 3 types: Type I (16 patients) periodic giant delta waves; Type II (10 patients) periodic giant delta waves intermixed with rapid spikes or fast activity; and Type III (6 patients), long spike-wave discharges interrupted by giant delta waves. Outcome score was associated with symptoms duration (P < 0.01) and EEG periodic complexes (P < 0.05). Symptom duration was inversely related to final outcome (multi-variable analysis). Long VSEEG monitoring greatly improves early diagnosis and detection of subtle atonia or segmental myoclonus. Prognostic factors were the type of EEG periodic complexes and duration of symptoms at presentation.