We report on the prenatal diagnosis of a fetus at risk of leprechaunism. We had previously determined the nature of the causative mutation in the insulin receptor gene in this family. The mutation removes a restriction site for the enzyme Mbo II. Genomic DNA was extracted from a chorionic villus sample and the 3' half of exon 2 was amplified by the polymerase chain reaction (PCR) followed by restriction digest. Using this method, we correctly predicted an unaffected child.