The recent US decline in sudden infant death syndrome (SIDS) rates may be explained by a shift in how these deaths are classified or reported. To examine this hypothesis, the authors compared cause-specific mortality rates for SIDS, other sudden, unexpected infant deaths, and cause unknown/unspecified, and they evaluated trends in the age and month of death for these causes using 1989–2001 US linked birth/death certificate data. Reported deaths in state and national data were compared to assess underreporting or overreporting. SIDS rates declined significantly from 1989–1991 to 1995–1998, while deaths reported as cause unknown/unspecified and other sudden, unexpected infant deaths, such as accidental suffocation and strangulation in bed (ASSB), remained stable. From 1999–2001, the decline in SIDS rates was offset by increasing rates of cause unknown/unspecified and ASSB. Changes in the cause-specific age at death and month of death distributions suggest that cases once reported as SIDS are now being reported as ASSB and cause unknown/unspecified. Most of the decline in SIDS rates since 1999 is likely due to increased reporting of cause unknown/unspecified and ASSB. Standardizing data collection at death scenes and improving the reporting of cause of death on death certificates should improve national vital records data and enhance prevention efforts.