Fifteen cases of congenital complete heart block were diagnosed in utero or at birth, over a nine-year period. Six cases were confirmed by fetal echocardiography between 22 and 36 weeks of gestation. Two cases were treated in utero for cardiac failure, and serial ultrasonography was used to monitor the progress. Structural heart disease was present in seven (47%) infants; in four of these infants the defect was a corrected transposition of the great arteries. Ventricular pacing was required in seven patients--four infants with anatomically-normal hearts and three infants with associated structural heart disease. Eight children did not require permanent pacing, and remained well after a mean follow-up period of 23 months (range, one to 72 months). The mortality was highest in those patients with structural heart disease, three of whom died as neonates. Only one baby died with an anatomically-normal heart. Antinuclear antibodies were present in the mothers of seven of the eight infants who were born with a structurally-normal heart. Four mothers either had pre-existing connective-tissue disease or had developed manifestations of such disease subsequent to pregnancy. Of this group, all the women were antibody-seropositive to the nuclear antigen SS-A during pregnancy. Mothers of three infants had antinuclear antibodies in low titres, were seronegative for antibody to SS-A and remained free of the manifestations of connective-tissue disease. Six of the seven most-recent cases have been diagnosed in utero, which confirms a trend towards the more-frequent prenatal diagnosis of congenital heart disease as a result of the increased use of fetal monitoring and ultrasonography.