There were 66,974 births at the three largest Dublin maternity hospitals in the years 1980-1982. Data on numbers of spontaneous and elective births, birthweight and perinatal mortality were analysed by day of the week. Perinatal mortality rates were highest on Wednesdays and Saturdays. The rate on Sunday was close to average. The largest number of perinatal deaths per day occurred on Wednesdays and the smallest on Sundays. Significant variations in mortality rates and percentage low birthweight were found among the 19% of infants who were born electively, but not among those born after a spontaneous onset of labour. High-risk pregnancies, including many with intra-uterine fetal death, were induced in large numbers from Tuesdays through Saturdays with a peak on Wednesdays. Low risk cases were induced mainly from Monday through Fridays with a peak on Fridays. The pattern of perinatal mortality through the week followed closely that of the risk status of pregnancies delivered electively. The results indicate that the pattern of perinatal mortality by day of the week of birth was determined by a highly organized weekly routine of selective elective delivery.