A total of 146 twin pregnancies were studied to evaluate the importance of bed rest. A significantly reduced frequency of preterm delivery was found after bed rest in hospital as compared to bed rest at home or no bed rest at all. An increase in gestational age and in birth weight was seen after bed rest. No effect was found on the intrauterine increase of fetal weight per time. Early routine ultrasound is recommended, so that effective bed rest may be instituted in due time.