Sleep polygraphic recording was carried out on 57 normal infants and on 100 SIDS siblings during morning naps between birth and the 4th month of life. Total sleep time and duration of sleep stages were determined. Central apnoeas of 2 sec and longer duration were analysed in AS, QS and IS. Apnoea index and number of apnoeas per 100 min of sleep stage were determined. Obstructive and mixed apnoeas were tabulated separately. Percentage of periodic breathing was also determined. Control babies and SIDS siblings were compared on these parameters, using the Mann-Whitney test. Between the 6th and 13th weeks of life respiratory pauses were significantly more frequent in SIDS siblings than in control subjects. The difference disappeared after the 13th week. The roles that peripheral afferents and the circadian organization of respiratory pauses play in determining the results are discussed. This technique does not appear to permit estimation of the risk of subsequent apnoeic episodes.