The hypothesis that poisoning by phosphines, arsines and stibines might be the primary cause of sudden infant death syndrome (SIDS) was investigated. Most mattress materials contain phosphorus or antimony compounds as fire retardant additives. Mattress materials in areas affected by the warmth and perspiration of the sleeping infant were found to be naturally infected by the fungus Scopulariopsis brevicaulis which is thought to be capable of generating phosphines, arsines and stibines from materials containing phosphorus, arsenic or antimony compounds. These gases may cause anticholinesterase poisoning and cardiac failure in infants, but contributory factors include the prone sleeping position and overwrapping. In England and Wales, the progressive increase in SIDS between 1951 and 1988 seems to be related to increasing use of phosphorus and antimony compounds as fire retardents in cot mattresses.