We studied 366 pregnancies in 330 prospectively managed women with asthma to determine the effect of pregnancy on asthma. Asthma activity was assessed by (1) daily symptom and medication diaries and (2) monthly auscultation and spirometry during pregnancy and for 3 months post partum. At 3 months post partum, subjects were asked to assess the overall course of their asthma during pregnancy compared to the usual course for them, and the course of their asthma during the 3 months post partum compared to the asthma during pregnancy. Asthma worsened during pregnancy in 35% of the women, improved in 28%, and was unchanged in 33%. Based on diary-card analysis, asthma was significantly less frequent and less severe during the last 4 weeks of pregnancy than during any other gestational interval. In women whose asthma improved during pregnancy, diary-card analysis revealed a gradual improvement with progressive pregnancy, whereas in women whose asthma worsened during pregnancy, there was an increase in asthma symptoms during 29 to 36 weeks gestation. During labor and delivery, asthma symptoms occurred in 10% of women with approximately equal proportions of these women receiving either no treatment or inhaled bronchodilators; only two subjects required intravenous aminophylline. During the 3 months post partum, asthma reverted toward its prepregnancy course in 73% of women. In 34 subjects prospectively studied for two successive pregnancies, there existed a significant concordance between the asthma course during the first and second pregnancies. The mechanistic and clinical implications of these findings are discussed.