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The five studies presented in this thesis were all conducted in Sweden as population based case-control studies. Children with Down's syndrome were excluded. A total of 652 cases were encompassed in the studies. Exposure data were blindly extracted from standardized medical records. There was no association between prenatal exposure to ultrasound or diagnostic x-rays and childhood leukemia. A history of maternal lower genital tract infection significantly increased the risk of childhood leukemia. This association was especially evident in children diagnosed at four years or older or in infancy. Resuscitation with 100% oxygen with a facemask and bag directly postpartum was associated with increased risk of childhood lymphatic leukemia. Previously described exposure risks related to childhood leukemia could not be confirmed by these studies. However, this thesis indicates that events during pregnancy or the neonatal period are associated with increased risks of lymphatic and infant leukemia.