To determine whether pregnancy is associated with an increased rate of flare in patients with systemic lupus erythematosus (SLE), we prospectively studied 40 pregnancies in 37 women with SLE. The women were evaluated on a monthly basis in the Hopkins Lupus Pregnancy Center. Flare was designated using a previously developed quantitative definition, i.e., a change of greater than 1.0 in the physician's global assessment (scale of 0-3) since the preceding visit or during the last 93 days. Flare occurred in 24 (60%) of the pregnancies. Flares presented most commonly as constitutional symptoms, renal involvement, or involvement of skin or joints. Comparison of the rates of flare in the same patients after delivery and in nonpregnant SLE patients showed a significant increase in the rate of flare during pregnancy (P less than 0.001 and P less than 0.0001, respectively). We conclude that flare of lupus during pregnancy is common and occurs significantly more frequently than does flare in nonpregnant SLE patients or in the same patients after pregnancy.