This paper is concerned with the formation Of Sudden infant death syndrome (SIDS) rates over time. Because of differential numbers of births throughout the year, a new SIDS rate is developed that takes into account the changing number of infants at risk and in particular the changing age distribution throughout the year. Differences between this newly adjusted rate and the commonly used unadjusted rates will be presented. Data from Minnesota linked birth-death records from 1990-1998 will be used for analysis. Copyright (C) 2004 John Wiley Sons, Ltd.