Thirty-one asthmatic women recruited in an ante-natal clinic were followed during pregnancy and for six weeks of the puerperium. Subjectively 22 (69%) women considered their asthma to have improved, two were worse and in seven (22%) there was no change. Analysis of peak flow rate, symptoms score and bronchodilator use showed that in ten pregnancies there was improvement in the third trimester and that in 11 there was deterioration in the puerperium. The results suggest that in patients with mild or moderate asthma an improvement is likely to occur during pregnancy, particularly in the last trimester, but that in over one-third there may be a post-natal deterioration. Review of the literature suggests that severe asthmatics are at greater risk of deterioration, particularly late in pregnancy.