To determine the biological significance of high concentrations of non-haem iron in the livers of infants dying from sudden infant death syndrome (SIDS), liver samples were obtained at necropsy from 66 infants who died from SIDS and 28 control infants who died before 2.5 years of age. All were full term deliveries. Liver iron concentrations decreased rapidly with age in the two groups. Liver iron concentrations in the SIDS infants and controls were compared for those infants who died between 1 month and 1 year of age. The median liver iron concentration in the SIDS infants was 296 micrograms/g wet weight; significantly higher than the median of 105 micrograms/g in controls. There was an inverse relation between iron concentration and age in the two groups, but an analysis of covariance confirmed the significantly lower values in controls. The frequency (22%) of HLA-A3 in SIDS infants was similar to that expected for the United Kingdom population (25%) and does not implicate the gene for haemochromatosis as a cause of high liver iron concentrations. These findings show that the peak incidence of SIDS occurs when mean concentrations of iron in liver tissue are higher than at any other time of life. Although a primary causal connection seems unlikely, high tissue iron concentrations may lower resistance to infection and enhance free radical formation, leading to tissue damage.