Two hundred eighty six infants with Down syndrome have been studied. Infantile spasms have been identified in nine of them, in which background, EEG pattern and its evolution, modalities of treatment and its effects, neuroimaging and development course have been revised. None of these patients had either familiar or personal pathological antecedents. The pattern in the first EEG made was hypsarrhytmic in all cases except one which showed a multifocal paroxystical activity, with intermittent and bilateral bursts of spike-waves. The treatment first used was ACTH in four cases, valproate in three cases and phenobarbital in two cases (one of these associated with nitrazepam). The ACTH treatment was effective in seven infants, either was the first or the second choice. An infant in whom the first treatment with ACTH was not successful, responded to the association with valproate+clonazepam. One patient treated initially with phenobarbital+nitrazepam, having no response to different prescriptions, responded finally to the association of carbamacepin+vigabatrin. The first treatment with valproate or phenobarbital had no effect in all patients. The EEG pattern improved in all cases just after the treatment response. Normal tracing was found for a period of two months to three years. Cranial TC was performed to three infants showing one of them a discrete ventricular dilatation and periventricular calcifications that suggested tuberous sclerosis. It is important to point out that, although the good effect of therapy and EEG pattern normalization, the development is below what had been expected in children with Down syndrome. Behavioral problems have been found in seven (77.7%) of these children.(ABSTRACT TRUNCATED AT 250 WORDS)