We investigated the impact of a long interval between the birth of the first and the second twin on second twin perinatal mortality (PNM). National data in the Swedish Medical Birth Registry were used on 7533 second twins born in Sweden between 1973 and 1985. PNM as a function of the time interval between the births of the twins was studied in data from two time periods: during 1973-78 (n = 4008) and 1979-85 (n = 3525). During the first period, PNM was significantly higher at intervals of 30 min or more between the births of the twins than at shorter intervals (chi 2 = 11.1, p less than 0.001). When studied within broad birth weight classes, a significant trend was seen for twins weighing 1500-2499 g with an increasing interval (chi 2 = 8.1: p less than 0.01). A non-significant trend was also found for twins weighing less than 1500 g but none for twins weighing greater than or equal to 2500 g. During the second period, abdominal delivery of the second twin after vaginal delivery of the first twin was significantly more common than during the first period (2.0% vs 0.3%, chi 2 = 52.7, p less than 0.001). During the second period, the interdelivery interval had little impact on second twin PNM. The results of this study seem to indicate that with modern management of labor and delivery, as seen in Sweden since 1979, the interdelivery interval has little impact on second twin PNM.