Interburst intervals (IBIs) are quiescent periods of cerebral activity, which normally occur in the electroencephalograms (EEGs) of premature infants. Although it is generally felt that the duration of these intervals shorten with increasing conceptional age (CA), no systemic studies of IBIs have been done in a large group of normal premature infants with long-term follow-up and using multichannel routine EEGs. In this study, we measured the IBIs, using defined criteria, in 36 premature infants who were normal at 3 years. The IBIs were measured in 104 EEGs, obtained from these infants, using standard recording techniques. Mean and the maximum IBIs were calculated. IBI duration decreased with increasing CA, although this trend was not very prominent when very restrictive criteria for measurement of IBI length were used. Less restrictive criteria for defining an IBI led to trends which are similar to those of previous studies. Various clinical factors, such as mild encephalopathies, small intraventricular/subependymal hemorrhages, mild bronchopulmonary dysplasia, or patent ductus arteriosus did not significantly alter IBI durations. Comparison with other techniques of IBI measurement and recording are discussed. The longest period of continuous activity during a routine recording was also measured and was found to increase with increasing CA.