To study delivery outcome in women with asthma, using Swedish health registers. Women with asthma were identified in two ways: by information in interviews performed by midwives at the pregnant woman's first visit to antenatal care, and by linkage between a medical birth register and a hospital discharge register, identifying women who had been hospitalized for asthma and also had a delivery. Births between 1984 and 1995 were studied. An increased risk for preterm birth and low birth weight was seen, possibly co-varying with disease severity. Also a significant increase in pregnancies of more than 41 weeks duration was noticed. An increase in infant death but not in congenital malformations rate was observed. An association with preeclampsia, gestational diabetes, and infant hypoglycemia was verified. Maternal asthma appears to be a risk factor for preterm and postterm births and increases the risk for some pregnancy complications.