Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a recessively inherited defect in the mitochondrial beta-oxidation of fatty acids. A single nucleotide change, the A985 --> G transition, in the MCAD gene accounts for approximately 90% of all the disease-causing mutations in the patients. We have used PCR to amplify a segment of the human MCAD gene and typed the allelic sequence variation at base 985 by a colorimetric oligonucleotide ligation assay (OLA). PCR/OLA provides a technique that permits differentiation of the homozygotes, heterozygotes, and normals for the A985 --> G allele in the MCAD gene. Genotyping of 1908 random Finnish DNA samples by OLA identified 10 carriers of the mutant allele, but no homozygotes were found. The calculated carrier frequency for the A985 --> G mutation was 1:191 (95% confidence limits, 1:118-1:501), and the calculated frequency for the A985 --> G homozygotes was 1:147,000 (95% confidence limits, 1:56,000-1:1,004,000).