Postmortem vitreous humor concentrations reflect antemortem serum chemical values. The authors measured the postmortem vitreous humor concentrations of Na+, K+, Cl-, Ca+2, Mg+2, urea nitrogen, creatinine, and total protein of 127 children who died from sudden infant death syndrome (SIDS) and other causes. Forty-seven children, 1 hour to 13 years old, had died of medical or surgical causes; 21 children, 7 weeks to 11 years old, had died following acute trauma; 59 children, 8 days to 1 year old, had died of SIDS. There was no significant difference between mean postmortem vitreous humor concentrations of those who died of medical and surgical causes and those who died after acute trauma (non-SIDS). in both groups, the mean postmortem vitreous humor concentrations of Mg+2 and Ca+2 were significantly higher in premature infants. There was direct correlation of postmortem vitreous humor concentration and postmortem interval for K+, but the variation, +/- 26 hours, was too large to be of practical importance in estimating time of death in individual cases. The three patient groups were compared and the mean postmortem vitreous humor concentrations of all eight constituents in the SIDS and non-SIDS groups were in the same ranges. In SIDS, the mean postmortem vitreous humor concentrations of Mg+2, Cl-, and urea nitrogen were significantly different from values of the non-SIDS cases, but not enough to indicate SIDS or to be informative about the etiology of SIDS. Measurement of the concentrations of postmortem vitreous humor constituents may not aid in the diagnosis of SIDS, but may aid in discovering unsuspected antemortem serum chemical abnormalities.