The incidence of sudden infant death syndrome (SIDS) in Denmark varied in the period 1982-1991 between 1.5 and 1.9 per 1000 livebirths. In December 1991 recommendations concerning infants' sleeping position were published by The Danish National Board of Health in order to reduce the risk of SIDS. Babies were recommended to be placed in the supine or side position when sleeping. Parents have followed the guidelines. Most Danish infants are now sleeping on their back or in the side position. Simultaneously, the number of SIDS dropped from about 110 to 40 per year. The incidence decreased to 1.2 in 1992 and was further reduced in 1993 to 0.6 per 1000 live births. Referring to our knowledge of the infant's temperature regulation we discuss why the prone position is a risk factor for SIDS. The head is the site of up to 85% of heat loss in an infant in bed. Placed in the prone position, the infant is more likely to suffer a rise in body temperature, especially if this is combined with having a cold, being heavily wrapped and sleeping in a heated room. Preceding sudden death many infants are reported to have suffered from minor viral infections. These might per se increase the body temperature. Parents often wrap infants that have an infection too heavily, which in an infant sleeping in the prone position might increase the body temperature to a higher level than if sleeping supine. The body temperature influences the production of toxins from normal intestinal flora and from pathogenic bacteria.(ABSTRACT TRUNCATED AT 250 WORDS)