To determine the feasibility and effectiveness of prenatal therapy of congenital heart block (CHB), information was sought regarding the timing of identification of CHB in 72 pregnancies and the outcome of those pregnancies in which the mothers were administered corticosteroids. Mailed questionnaires with telephone follow-up, data from primary physicians and chart review were utilized. In 38 (53%) of affected pregnancies CHB was identified between 16 and 24 weeks of gestation, in 17 (24%) between 25 and 30 weeks of gestation, in 8 (11%) between 31 and 37 weeks, and in 5 (7%) between 38 and 40 weeks. In four pregnancies the timing was unknown. Five women were taking prednisone for disease activity prior to the discovery of CHB and in six other women prednisone therapy was initiated at or about the time CHB was identified. In 19 pregnancies, women were given fluorinated steroids (available to the fetus in an active form) as attempted therapy after the discovery of CHB. Of these, one fetus with second degree block reverted to sinus rhythm and two with third degree block exhibited an improvement in the degree of block. In eight fetuses pleural and/or pericardial effusions resolved. In conclusion, the gestational period of heightened fetal vulnerability for the development of heart block is in the mid second to early third trimester. Although there is no evidence that maternal prednisone should be used prophylactially, fluorinated steroids may be efficacious after the identification of heart block, particularly with regard to an associated myocarditis.