The presenting characteristics of 18 (11 female, seven male) children with Landau-Kleffner syndrome (LKS) were studied with respect to course and outcome of their condition at a mean length of 67 (SD 46) months' follow-up. All had regression of receptive language (mean age of onset 4 years 9 months) and electrical status epilepticus in sleep (ESES). Length of ESES correlated strongly with length of period between onset of illness and onset of recovery (p<0.006) and also with eventual receptive (p<0.001) and expressive (p<0.007) language. Behaviour during the acute phase was severely affected in nine children and associated with frontal lobe discharges in awake EEGs (p<0.004). Age at onset was not correlated with outcome. All children had impaired short-term memory at follow-up. Three children had language outcome within the normal range. No child with ESES lasting longer than 36 months had normal language outcome. These data lend support for intervention in ending ESES by 36 months using multiple subpial transection (MST) if steroids are ineffective or cause unacceptable side effects.