In 12 adults with typical Lennox-Gastaut syndrome and partial epilepsies with secondary bilateral synchrony unsatisfactorily controlled by current antiepileptic drugs, the following computerized background EEG parameters were studied before and during beneficial antiepileptic effect of lamotrigine addition: absolute and relative spectral power density; alpha/theta index; dominant frequency of occipital alpha, theta and delta bands. The only significant influence of lamotrigine addition was a moderate decrease of the median and mean absolute delta power (p < 0.01). We concluded there was a poor influence of therapeutic doses of lamotrigine on the background diffuse slow dysrhythmias characteristic to severe early encephalopathies which engender both severe secondary epilepsies and different degrees of mental handicap.