The incidence of alpha-thalassemia trait (alpha-thal-1 and alpha-thal-2) among Southern Chinese in Hong Kong is about 3%. From June 1983 to September 1987, prenatal diagnosis for homozygous alpha-thal-1 was performed in 88 pregnancies at risk, using direct DNA analysis of amniotic fluid cells or chorionic villi. Twenty-one homozygous alpha-thal-1 fetuses were aborted and confirmed as Hb Bart's hydrops fetalis, and two were "Hb H" hydrops fetalis. Of 47 pregnancies delivered, 26 were alpha-thal-1 heterozygotes, 10 normal, eight alpha-thal-1/normal. Twenty-one pregnancies, diagnosed as alpha-thal-1/normal, await delivery. Based on a 6% incidence of beta-thalassemia minor among pregnant women in Hong Kong, the number of pregnancies at risk for beta-thalassemia major should be 288 per annum. However, since February 1984, only 25 diagnoses were performed. Comprehensive screening and education programs need to be implemented. The majority of beta-thalassemia defects in Southern Chinese are point mutations, single nucleotide insertions or minor deletions, not detectable by standard gene mapping techniques. With linkage analysis of the defective gene to polymorphic restriction sites, a definitive diagnosis can be obtained in 50% of the families, while in the remaining there is a 50% exclusion rate. We routinely use the Hind III-3' beta, Bam HI-3' beta, Ava II-beta, and Hinc II-psi beta sites for linkage analysis. For first pregnancies, the marked linkage disequilibrium of the Bam HI polymorphism can be applied in 29% of the cases. In nonconclusive cases, fetal blood beta/gamma globin chain synthetic ratio was used.(ABSTRACT TRUNCATED AT 250 WORDS)