Previous reports suggest an association between maternal serum complement component (C3 and C4) levels and perinatal outcome. Eighteen women with systemic lupus erythematosus (21 pregnancies) were prospectively studied. C3 and C4 levels were measured monthly unless more frequent data were needed for acute medical management. Perinatal outcome was then correlated with both maternal clinical and serologic status before and during pregnancy. Twelve pregnancies were carried to term, 11 of which began in clinical remission. Eight of these 12 had normal C3 and C4 levels at the onset of pregnancy, and 11 were normal at delivery. Four pregnancies were delivered prematurely (with one neonatal death from septic meningitis); all began pregnancy in clinical remission but three had low C3 and C4 levels before delivery. Three pregnancies beginning in clinical remission ended in spontaneous first- or early second-trimester abortions; all had low C3 and C4 levels before aborting. Normal complement component levels were observed in the remaining two pregnancies that were terminated electively; they had no evidence of fetal or placental abnormality. The correlation of maternal serum complement levels and pregnancy outcome in this series suggests that these parameters may help in the assessment of fetal as well as maternal prognosis in lupus-complicated gestations.