Hormonal measurements and HLA genotyping of amniotic fluid at midgestation correctly predicted the postnatal diagnosis of congenital adrenal hyperplasia (CAH) in 29 of 32 fetuses at risk. Of these 29, 7 were predicted to have prenatal-onset classical 21-hydroxylase deficiency (21-OH def) based on elevated amniotic fluid 17-hydroxyprogesterone (17-OHP) and delta 4-androstenedione (delta 4-A) levels. These 7 fetuses and their index cases were ultimately proven to have salt-wasting classical 21-OH def. Of 5 who were HLA typed, the genotype was identical to the index case in 4; in one, HLA prediction was not possible, because the parents shared identical HLA antigens. Normal amniotic fluid 17-OHP and delta 4-A levels in the remaining 22 fetuses predicted that they were not affected with classical CAH. These children have been clinically asymptomatic to date or proven biochemically not to have classical or nonclassical CAH. Of the 22 fetuses, 11 were predicted by HLA genotyping to be homozygous normal or heterozygous for 21-OH def. In 3 of the 32 fetuses, prenatal diagnosis was incorrect. In one, the fetus was predicted to have CAH based on HLA identity to the index case. However, amniotic fluid 17-OHP and delta 4-A were normal, and the fetus was normal. The index case of this family did not have CAH, but was a normal child. Thus, amniotic fluid hormone levels accurately predicted a normal fetus, while HLA genotyping was not relevant in prenatal diagnosis because the index case was unaffected. The second fetus was predicted to be affected on the basis of HLA genotyping and to be unaffected based on normal amniotic fluid 17-OHP and delta 4-A. During infancy, this female infant had postnatal-onset nonclassical CAH. The index case in this family, presumed to have classical simple virilizing CAH, was later diagnosed to have nonclassical CAH. Thus, in nonclassical CAH, hormonal measurement of 17-OHP and delta 4-A is not useful in prenatal diagnosis; only HLA genotyping of the fetus is valuable. In the third case, the fetus was predicted to be a heterozygote by HLA genotyping and to be unaffected by hormonal measurement. Postnatally, at age 2 7/12 yr, the male child was found to have classical simple virilizing CAH and to be HLA-B-DR identical to his brother (index case) who also has classical simple virilizing CAH.(ABSTRACT TRUNCATED AT 400 WORDS)