The objective was to determine whether the frequency of flare in systemic lupus erythematosus (SLE), patients is increased during pregnancy and the puerperium. Seventy-eight pregnancies in 68 SLE patients attending the lupus pregnancy clinic, at St. Thomas' Hospital, during the last 5 yr were included. The pregnancy period and 8 weeks post-delivery were considered. This group was compared with a control group of 50 consecutive, non-pregnant, age-matched SLE patients attending our weekly lupus clinic. Additionally, 43 of the pregnant patients carried on attending the lupus clinic for the year after puerperium, and their course was compared with themselves during pregnancy. SLE activity was assessed using the Lupus Activity Index (LAI) score. An increase > or = 0.26 in the score was considered as a flare of the disease. Pregnancy and control groups were homogeneous for age, race, disease duration and distribution of autoantibodies. Sixty-five per cent of the patients flared during pregnancy and/or the puerperium and 42% flared in the control group (P = 0.015). The rates of flare per patients/month were 0.082 +/- 0.004 for the pregnancy group and 0.039 +/- 0.003 for the control group (P < 0.001). The 43 patients whose course was controlled after the puerperium flared more frequently during pregnancy that thereafter (McNemar test, P = 0.003). The rates of flare per patient/month were 0.093 +/- 0.006 during pregnancy and the puerperium, and 0.049 +/- 0.004 after the puerperium (P = 0.0015). Kidney and central nervous system involvement was not different between the pregnancy and control groups. In terms of frequency of flares, there was no difference in any of the groups between patients taking and not taking steroids. We conclude that SLE tends to flare during pregnancy. Flares are maximal during the second and third trimester and the puerperium. Flares are not more severe than in non-pregnant patients, and most of the flares can be managed conservatively. Prednisolone does not prevent flares.