Post-mortem hypoxanthine concentrations in the vitreous humor of human infants were investigated. Hypoxanthine is formed from hypoxic degradation of adenosine monophosphate. The concentrations in the vitreous humor can give information about antemortem hypoxia. The post-mortem levels were corrected for the time elapsing between death and the autopsy. Four groups of infants were compared: 17 babies who died of respiratory distress syndrome (RDS), 72 infants who died of sudden infant death syndrome (SIDS), 23 children dying of congenital heart disease (both cyanotic and acyanotic), and 15 children dying acutely in accidents without any known significant time of hypoxia before death. The corrected, median hypoxanthine levels in victims of SIDS (200 mumol/L) was significantly higher (p < 0.01) than in the accident group (0 mumol/L), but no clear difference was found between the SIDS group and the RDS group (101 mumol/L), or the heart group (54 mumol/L). A number of children with "normal" hypoxanthine levels (0 to 38 mumol/L) were found in all four groups, but the numbers were significantly lower (p < 0.005) in the RDS, SIDS and heart groups than in the accident group. It is concluded that SIDS is probably not a sudden event, but may be preceded by relatively long, or repeated intermittent periods of hypoxia (of unknown etiology).