In the X-linked form of chronic granulomatous disease (X91 degrees CGD), the genetic defect is linked to the CYBB locus on the X chromosome. We studied a family with a genetic defect in this gene, consisting of a G----A substitution at the fifth base of the 5' donor splice site of intron 3. This mutation leads to skipping of exon 3 after transcription of the gene. The expectant mother was diagnosed as a carrier. Analysis of polymerase chain reaction (PCR)-amplified genomic DNA from a chorionic villus biopsy (CVB) showed the same mutation in the male fetus. After termination of the pregnancy, the diagnosis was confirmed by conventional methods. This is the first time that PCR has been used for prenatal diagnosis of CGD.