Since maturation of ABR amplitude and latency in principle are dependent on innate as well as environmental factors and the health of the infant, it is conceivable that prematurely born infants show a maturation profile that is different from that of full term infants. In order to unequivocally address this we analyzed ABRs recorded in 465 full term infants and in 179 healthy prematurely born, of which 65 were tested once and 113 were tested (2-9 occasions) at different ages from birth up to 5 years of age. Our emphasis was on a description in terms of time constants of the exponential functions used to describe the maturational changes. In general one exponential function (time constant about 5 weeks) was needed to describe the changes in full term wave I latency. The sum of two exponentials was required to describe adequately the latency changes for full term wave V (time constants about 5 weeks and 100 weeks). In prematurely born infants all time constants for the wave I and wave V changes were longer than in the full term. For the wave I-wave V interval no difference was found between the two populations. The differences in the time course for wave I and wave V latency can be accounted for by the higher incidence of otitis media in the prematurely born group.